<PAGE>   1
                                                                    EXHIBIT 99.1


                      [INVESTORSBANCORP, INC. LETTERHEAD]



                                  April 6, 1999





Dear Shareholder:

         On behalf of the Board of Directors and management of InvestorsBancorp,
Inc., we cordially invite you to attend the Annual Meeting of Shareholders of
InvestorsBancorp Inc., to be held at 5:00 p.m. on Thursday, May 6, 1999, in the
South Grand Ballroom of the Milwaukee Athletic Club, 758 North Broadway,
Milwaukee, Wisconsin. The accompanying Notice of Annual Meeting of Shareholders
and Proxy Statement discuss the business to be conducted at the meeting. A copy
of the Company's Form 10-KSB is also in this booklet. At the meeting we shall
report on Company operations and the outlook for the year ahead.

         Your Board of Directors has nominated two persons to serve as Class II
directors, each of whom are incumbent directors. The Company's Board of
Directors has selected and recommends that you ratify the appointment of
Virchow, Krause & Company LLP (the successor firm to Conley, McDonald LLP) to
continue as the Company's independent public accountants for the year ending
December 31, 1999.

         We recommend that you vote your shares for the director nominees and in
favor of the proposal.

         We encourage you to attend the meeting in person. WHETHER OR NOT YOU
PLAN TO ATTEND, HOWEVER, PLEASE COMPLETE, SIGN AND DATE THE ENCLOSED PROXY AND
RETURN IT IN THE ACCOMPANYING POSTPAID RETURN ENVELOPE AS PROMPTLY AS POSSIBLE.
This will ensure that your shares are represented at the meeting.

          We look forward with pleasure to seeing and visiting with you at the
meeting.

                                         Very truly yours,

                                         INVESTORSBANCORP, INC.



                                         George R. Schonath
                                         President and Chief Executive Officer



<PAGE>   2




                      [INVESTORSBANCORP, INC. LETTERHEAD]






                    NOTICE OF ANNUAL MEETING OF SHAREHOLDERS
                            TO BE HELD ON MAY 6, 1999
                    ----------------------------------------

                     
TO THE HOLDERS OF COMMON STOCK OF INVESTORSBANCORP, INC.

         Notice is hereby given that the Annual Meeting of Shareholders of
InvestorsBancorp, Inc. (the "Company"), will be held in the South Grand Ballroom
of the Milwaukee Athletic Club, 758 North Broadway, Milwaukee, Wisconsin, on
Thursday, May 6, 1999 at 5:00 p.m., for the purpose of considering and voting
upon the following matters:

         1.       the election of two (2) Class II directors of the Company.

         2.       the ratification of the appointment of Virchow, Krause &
                  Company LLP as auditors for the Company for the fiscal year
                  ending December 31, 1999.

         3.       the transaction of such other business as may properly come
                  before the meeting or any adjournments or postponements
                  thereof.

         The Board of Directors is not aware of any other business to come
before the meeting. Shareholders of record at the close of business on March 22,
1999, are the shareholders entitled to vote at the meeting and any adjournments
or postponements thereof.

                                          By Order of the Board of Directors



                                          George R. Schonath
                                          President and Chief Executive Officer


Pewaukee, Wisconsin
April 6, 1999


IMPORTANT:  THE PROMPT RETURN OF PROXIES WILL SAVE THE COMPANY THE EXPENSE OF
FURTHER REQUESTS FOR PROXIES TO ENSURE A QUORUM AT THE MEETING.  A SELF-
ADDRESSED ENVELOPE IS ENCLOSED FOR YOUR CONVENIENCE.  NO POSTAGE IS REQUIRED
IF MAILED WITHIN THE UNITED STATES.


<PAGE>   3






                             INVESTORSBANCORP, INC.




                                 PROXY STATEMENT


         This Proxy Statement is furnished in connection with the solicitation
on behalf of the Board of Directors of InvestorsBancorp, Inc. (the "Company") of
proxies to be used at the meeting which will be held in the South Grand Ballroom
of the Milwaukee Athletic Club, 758 North Broadway, Milwaukee, Wisconsin, on
Thursday, May 6, 1999 at 5:00 p.m., and all adjournments or postponements of the
meeting. The Proxy Statement and the accompanying Notice of Meeting and Proxy
are first being mailed to holders of shares of common stock, par value $.01 per
share, of the Company (the "Common Stock") on or about April 6, 1999. Certain of
the information in the Proxy Statement relates to InvestorsBank, a Wisconsin
chartered bank located in Pewaukee, Wisconsin (the "Bank"), the wholly owned
subsidiary of the Company.

VOTING RIGHTS AND PROXY INFORMATION

         All shares of Common Stock represented at the meeting by properly
executed proxies received prior to or at the meeting, and not revoked, will be
voted at the meeting in accordance with the instructions thereon. If no
instructions are indicated, properly executed proxies will be voted for the
nominees and for adoption of the proposals set forth in this Proxy Statement. A
majority of the shares of the Common Stock, present in person or represented by
proxy and entitled to vote, shall constitute a quorum for purposes of the
meeting. Abstentions and broker non- votes will be counted for purposes of
determining a quorum.

         Only holders of record of the Common Stock at the close of business on
March 22, 1999 will be entitled to vote at the meeting and at all adjournments
or postponements of the meeting. On March 22, 1999, the Company had 1,000,000
shares of Common Stock outstanding. Directors are elected by a plurality of the
votes cast in person or by proxy with a quorum present. For all other matters,
the affirmative vote of a majority of the votes cast in person or by proxy with
a quorum present shall constitute shareholder approval. Abstentions and broker
"non-votes" will be considered in determining the presence of a quorum but will
not affect the vote required for approval of the election of directors or any
proposal.

         The Company does not know of any matters, other than as described in
the Notice of Meeting, that are to come before the meeting. If any other matters
are properly presented at the meeting for action, the persons named in the
enclosed form of proxy and acting thereunder will have the discretion to vote on
such matters in accordance with their best judgment.

         The Board of Directors would like to have all shareholders represented
at the meeting. Whether or not you plan to attend, please complete, sign and
date the enclosed proxy and return it in the accompanying postpaid return
envelope as promptly as possible. A proxy given pursuant to this solicitation
may be revoked at any time before it is voted. Proxies may be revoked by: (i)
duly executing and delivering to the Secretary of the Company a later dated
proxy relating to the same shares prior to the exercise of such proxy, (ii)
filing with the Secretary of the Company at or before the meeting a written
notice of revocation bearing a later date than the proxy, or (iii) attending the
meeting and voting in person (although attendance at the meeting will not in and
of itself constitute revocation of a proxy). Any written notice revoking a proxy
should be delivered to Mr. Jon McGlocklin, Secretary, at W239 N1700 Busse Road,
P.O. Box 190, Pewaukee, Wisconsin 53072.



<PAGE>   4



                              ELECTION OF DIRECTORS

         At the Annual Meeting of the Shareholders to be held on Thursday, May
6, 1999, the shareholders will be entitled to elect two (2) Class II directors
for a term expiring in 2002. The directors of the Company are divided into three
classes having staggered terms of three years. Both of the nominees for election
as Class II directors are incumbent directors. The Company has no knowledge that
either of the nominees will refuse or be unable to serve, but if any of the
nominees becomes unavailable for election, the holders of the proxies reserve
the right to substitute another person of their choice as a nominee when voting
at the meeting.

         Set forth below is information concerning the nominees for election and
for the other directors whose terms of office will continue after the meeting,
including the age, year first elected a director and business experience of each
during the previous five years. The nominees, if elected at the Annual Meeting
of Shareholders, will serve as Class II directors for three year terms expiring
in 2002. THE BOARD OF DIRECTORS RECOMMENDS THAT SHAREHOLDERS VOTE FOR EACH OF
THE NOMINEES FOR DIRECTOR.


                                 NOMINEES

                                                                DIRECTOR OF THE
                         PRESENT POSITION WITH THE              COMPANY OR THE
NAME AND AGE             COMPANY AND THE BANK                   BANK SINCE
------------             -------------------------              ---------------

CLASS II
--------
(Term Expires 2002)
Salvatore L. Bando       Director of the Company and the             1997
(Age 54)                 Bank

Terry L. Mather          Director of the Company and the             1997
(Age 56)                 Bank

                           CONTINUING DIRECTORS
CLASS III
(Term Expires 2000)
Donald E. Sydow          Director of the Company and the             1997
(Age 63)                 Bank

CLASS I
(Term Expires 2001)
George R. Schonath       Director, President and Chief               1997
(Age 57)                 Executive Officer of the Company
                         and the Bank

Jon McGlocklin           Director, Senior Vice President and         1997
(Age 55)                 Secretary of the Company and
                         Director and Senior Vice President
                         of the Bank



         All of the Company's directors will hold office for the terms
indicated, or until their respective successors are duly elected and qualified.
There are no arrangements or understandings between the Company and any other
person pursuant to which any of the Company's directors have been selected for
their respective positions.


                                        2

<PAGE>   5



BIOGRAPHICAL DATA

         The principal occupation of each director is set forth below. Each
director has held his present position for at least five years unless otherwise
indicated.

         SALVATORE L. BANDO has served as Senior Vice President of the Baseball
Operations for the Milwaukee Brewers since 1991. Mr. Bando also served as a
director of Bando McGlocklin Capital Corporation, a real estate investment trust
located in Pewaukee, Wisconsin ("BMCC"), from 1980 until 1997, and was an
officer of that company from 1980 until 1991.

         TERRY L. MATHER has been a partner of Critical Solutions, Inc., a
business consulting firm headquartered in Milwaukee, Wisconsin, since 1992.

         JON MCGLOCKLIN has been a Senior Vice President of the Company and the
Bank since they were established in 1997. He has also served as President of
Healy Manufacturing, Inc., Menomonee Falls, Wisconsin, since 1997, and as an
announcer for the Milwaukee Bucks since 1976. In July, 1997, Mr. McGlocklin was
appointed Senior Vice President of BMCC. Until July, 1997, he served as a
director (since 1980) and President (since 1991) of BMCC.

         GEORGE R. SCHONATH has been the President and Chief Executive Officer
of the Company and the Bank since they were established in 1997. In July, 1997,
Mr. Schonath was appointed President of BMCC, and also currently serves as its
Chief Executive Officer (since 1983). Until July, 1997, he served as a director
(since 1980) and Chairman of the Board (since 1983) of BMCC.

         DONALD E. SYDOW has been the President of Oconomowoc Manufacturing
Corp., a ball-bearing manufacturer located in Oconomowoc, Wisconsin, since 1982.

MEETINGS OF THE BOARD OF DIRECTORS AND COMMITTEES

         During 1998, the Board of Directors of the Company held one meeting,
while the Bank's Board of Directors held four meetings. No director attended
fewer than 75% of the total number of meetings of the Board of Directors of the
Company or the Bank held during 1998. The Company's Board of Directors has
appointed an Audit and Finance Committee and a Compensation Committee.

         The Audit and Finance Committee consists of Messrs. Bando, Mather and
Sydow. The Audit and Finance Committee reviews audit reports and related matters
to ensure effective compliance with regulations and internal policies and
procedures. This committee also recommends to the Board the accounting firm to
perform the Company's and Bank's annual audit and acts as the liaison between
the auditors and the Board. The committee also is responsible for reviewing,
approving and recommending to the Board financial policies and strategies of the
Bank and significant expenditures proposed to be made by the Company or the
Bank. During 1998 the Audit and Finance Committee did not meet.

         The Compensation Committee consists of Messrs. Bando, Mather and Sydow.
The Compensation Committee meets to review the performance, salary and other
compensation of the Chief Executive Officer officers of the Company and make
recommendations to the Board, including with respect to stock options and other
incentive awards. During 1998 the Compensation Committee met two times.

COMPENSATION OF DIRECTORS

         Directors of the Company who are not also officers receive a fee of
$5,000 per year and $750 for each Board or committee meeting attended. Directors
who are also officers of the Company do not receive additional compensation for
their service as directors of the Company.


                                        3

<PAGE>   6

         SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

         The following table sets forth certain information with respect to the
beneficial ownership of the Company's Common Stock at March 1, 1999, by each
person known by the Company to be the beneficial owner of more than five percent
of the outstanding Common Stock, by each director or nominee, by each executive
officer named in the Summary Compensation Table, and by all directors and
executive officers of the Company as a group.



NAME OF INDIVIDUAL AND                  Amount and Nature of          PERCENT
NUMBER OF PERSONS IN GROUP              Beneficial Ownership(1)       OF CLASS
--------------------------              -----------------------       --------

DIRECTORS AND 5% SHAREHOLDERS

Salvatore L. Bando                            37,395(2)                    3.7%

Terry L. Mather                                   --                         *

Jon McGlocklin                                33,617(3)                    3.4%

George R. Schonath                           363,808(4)                   33.1%

Donald E. Sydow                                4,001                       0.4%

ALL DIRECTORS AND EXECUTIVE OFFICERS
AS A GROUP (8 persons)                       455,181(5)                   41.2%


-------------------------------
* Less than one percent.


(1) Information contained in this column is based upon information furnished to
the Company by the persons named above and the members of the designated group,
except as set forth in the footnotes below. The nature of beneficial ownership
for shares shown in this column is sole voting and investment power, except as
set forth in the footnotes below. Inclusion of shares shall not constitute an
admission of beneficial ownership or voting and investment power over included
shares. Pursuant to the rules and regulations of the Securities and Exchange
Commission and the Securities Exchange Act of 1934, as amended (the "Exchange
Act"), share amounts include shares obtainable through the exercise of stock
options or warrants within 60 days of the date of the information contained in
this table.

(2) Includes 12,613 shares over which Mr. Bando shares voting and investment
power with his spouse.

(3) Includes 10,093 shares over which Mr. McGlocklin shares voting and
investment power with his spouse.

(4) Includes 100,000 shares obtainable through the exercise of warrants, over
which shares Mr. Schonath has no voting and sole investment power, 1,970 shares
over which Mr. Schonath shares voting and investment power with his spouse or
children and 28,650 shares held by Lake Country Investments, LLC, a limited
liability company, over which Mr. Schonath has shared voting and investment
power with the Schonath Family Partnership, a limited Partnership controlled by
Mr. Schonath. Excludes shares held for other employees by the Company's 401(K)
Plan, for which Mr. Schonath is a co-trustee and may be deemed to have shared
voting power.

(5) Includes 103,750 shares obtainable through the exercise of options or
warrants. Excludes shares held for other employees by the Company's 401(K) Plan,
for which Mr. Schonath is a co-trustee and may be deemed to have shared voting
power.

         Section 16(a) of the Exchange Act requires the Company's executive
officers and directors and persons who own more than 10% of the Common Stock to
file reports of ownership and changes in ownership with the Securities and
Exchange Commission and with the exchange on which the shares of Common Stock
are traded. Such persons

                                        4

<PAGE>   7



are also required to furnish the Company with copies of all Section 16(a) forms
they file. Based solely upon the Company's review of such forms and, if
appropriate, representations made to the Company by any such reporting person
concerning whether a Form 5 was required to be filed for the 1998 fiscal year,
the Company is not aware that any of its directors and executive officers or 10%
shareholders failed to comply with the filing requirements of Section 16(a)
during the period commencing January 1, 1998 through December 31, 1998, except
that Donald E. Sydow, a director, failed to report a purchase of stock on a
timely basis and George R. Schonath, President and Chief Executive Officer,
filed a late Form 4 to report certain transactions that had been previously
reported in a timely manner under the name of the Schonath Family Partnership, a
limited partnership, instead of George R. Schonath.


                             EXECUTIVE COMPENSATION

         The following table sets forth information concerning the compensation
paid or granted by the Company or the Bank to the Company's Chief Executive
Officer since the Company's inception, and to each of the other executive
officers of the Company and the Bank whose aggregate salary and bonus exceeded
$100,000.

<TABLE>
<CAPTION>
====================================================================================================================================

                                                      SUMMARY COMPENSATION TABLE
------------------------------------------------------------------------------------------------------------------------------------

                                                                                         Long Term Compensation
                                                       Annual Compensation                       Awards
------------------------------------------------------------------------------------------------------------------------------------
             (a)                  (b)        (c)          (d)             (e)             (f)             (g)             (h)
                                                                                       Restricted      SECURITIES       ALL OTHER
Name and                                                              Other Annual        Stock        UNDERLYING      COMPENSATION
                                 Year      Salary($)    BONUS($)   Compensation($)(2)   Award($)    OPTIONS/SARS (#)       ($)
Principal Position
====================================================================================================================================
<S>                              <C>       <C>          <C>           <C>             <C>                   <C>           <C>
George R. Schonath,              1998      $ 100,000    $  ---        $ 23,089        $      ---            ---           $   [ ]
President and Chief Executive    1997         31,250       ---           7,536               ---            ---             2,511
Officer(1)                                                                                          
------------------------------------------------------------------------------------------------------------------------------------

Scott J. Russell,                1998      $ 107,500    $5,000        $    ---        $      ---            ---           $  ---
Senior Vice President
------------------------------------------------------------------------------------------------------------------------------------

Joel C. Obermeirer,              1998      $ 107,500    $5,000        $    ---        $      ---            ---           $  ---
Senior Vice President
====================================================================================================================================
</TABLE>

(1) Amounts paid are net of the amount reimbursed by BMCC pursuant to the
Management Services and Allocation of Expenses Agreement. Under this Agreement,
62.26% of Mr. Schonath's annual salary is paid by BMCC. Salary for 1997 is for
the period from the September 8, 1997 opening date of the Bank. Also includes
amounts deferred under the Company's 401(k) Plan.

(2) Represents the Bank's contribution for supplemental retirement benefits.



                                        5

<PAGE>   8




STOCK OPTIONS

         None of the executive officers of the Company listed in the summary
compensation table received any grant of stock options in the 1998 fiscal year
or held any such options as of December 31, 1998.

         The following table sets forth certain information concerning the stock
options at March 1, 1999 held by the named executive officers. No stock options
were exercised during 1998 by such persons.


<TABLE>
<CAPTION>
===================================================================================================================
                          AGGREGATED OPTION/SAR EXERCISES IN LAST FISCAL YEAR AND FY-END
                                                 OPTION/SAR VALUES
-------------------------------------------------------------------------------------------------------------------
                              Shares                         Number of Securities
                             Acquired                       Underlying Unexercised      Value of Unexercised In-
                                on          Value           Options/SARs at FY-End       the-Money Options/SARs
           Name              Exercise     Realized                  (#)(d)                  at FY-End ($)(e)
           (a)                (#)(b)       ($)(c)          Exercisable   Unexercisable Exercisable   Unexercisable

-------------------------------------------------------------------------------------------------------------------
<S>                          <C>          <C>              <C>              <C>          <C>            <C>
 George R. Schonath            ---        $   ---            ---              ---        $  ---         $
-------------------------------------------------------------------------------------------------------------------
 Scott J. Russel               ---        $   ---          1,250            1,250        $1,469         $1,469
-------------------------------------------------------------------------------------------------------------------
 Joel C. Obermeirer            ---        $   ---            500            2,000        $  586         $2,350
===================================================================================================================
</TABLE>

COMPENSATION COMMITTEE INTERLOCKS AND INSIDER PARTICIPATION

         During 1998, the full Board of Directors considered and approved the
compensation packages of the employees of the Company and the Bank. None of
these individuals was an officer or employee of the Company or the Bank during
1998, except for Messrs. McGlocklin and Schonath, and none of these individuals
is a former officer or employee of the Company or any of its subsidiaries.
Messrs. McGlocklin and Schonath do not participate in decisions regarding their
respective compensation. Messrs. Bando, McGlocklin and Schonath were directors
and officers of BMCC prior to the distribution by BMCC and its shareholders of
all of its shares of the Company.


                          TRANSACTIONS WITH MANAGEMENT

         Directors and officers of the Company and the Bank, and their
associates, were customers of and had transactions with the Company and the Bank
during 1998. The Bank and BMCC also purchase loan participations from each other
from time to time and, pursuant to a Management Services and Allocation of
Expenses Agreement, the Bank performs certain loan servicing and administration
services to BMCC. Additional transactions may be expected to take place in the
future. All outstanding loans, commitments to loan, transactions in repurchase
agreements and certificates of deposit and depository and loan participation and
servicing relationships, in the opinion of management, were made in the ordinary
course of business, on substantially the same terms, including interest rates
and collateral, as those prevailing at the time for comparable transactions with
other persons and did not involve more than the normal risk of collectibility or
present other unfavorable features. The Bank received an aggregate of $775,892
in loan servicing and administration fees from BMCC in 1998. The Company and the
Bank also sublease space for their main offices from BMCC. BMCC leased its
office space from Bando McGlocklin Real Estate Investment Corp. ("BMREIC") until
July 14, 1998, at which time Bando McGlocklin Small Business Lending
Corporation, a wholly owned subsidiary of BMCC, acquired the assets of BMREIC.
The annual rent under the sublease, including real estate taxes, utilities and
furnishings, is approximately $127,000, which represents a pro rata share of
BMCC's occupancy expense. The Company believes the terms of the sublease with
BMCC are on substantially the same terms and conditions as could be obtained
from unrelated third parties.


                                        6

<PAGE>   9



               RATIFICATION OF APPOINTMENT OF INDEPENDENT AUDITORS

         The Board of Directors of the Company has appointed Virchow, Krause &
Company LLP, independent accountants, to be the Company's auditors for the
fiscal year ending December 31, 1999, and recommends that the shareholders
ratify the appointment. Virchow, Krause & Company LLP, is the successor by
merger to Conley, McDonald LLP, which has been the Company's independent
auditors since December, 1997. A representative of Virchow, Krause & Company LLP
is expected to attend the Annual Meeting and be available to respond to
appropriate questions and make a statement if he or she so desires. If the
appointment is not ratified, the matter of the appointment of auditors will be
considered by the Board of Directors.

         In connection with the audit of the Company's financial statements from
September 3, 1997 through December 31, 1998, there were no unresolved issues,
scope restrictions, unanswered questions or disagreements with Virchow, Krause &
Company LLP on any matter of accounting principles or practices, financial
statement disclosure, or auditing scope or procedure which, if not resolved to
the satisfaction of Virchow, Krause & Company LLP, would have caused Virchow,
Krause & Company LLP to make reference to the matter in their report, and
Virchow, Krause & Company LLP did not advise the Company that any of the events
described in Item 304 (a)(1)(iv)(B) of Regulation S-B had occurred.

         During the Company's fiscal years ended December 31, 1998, the Company
(or anyone on the Company's behalf) did not consult Virchow, Krause & Company
LLP regarding: (i) either the application of accounting principles to a
specified transaction, either completed or proposed; or the type of audit
opinion that might be rendered on the Company's financial statements; and as
such no written report was provided to the Company and no oral advice was
provided that the accountant concluded was an important factor considered by the
Company in reaching a decision as to any accounting, auditing or financial
reporting issue, or (ii) any matter that was either the subject of disagreement
or a reportable event.


         THE BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT YOU VOTE FOR
RATIFICATION OF THE APPOINTMENT OF VIRCHOW, KRAUSE & COMPANY LLP.

                              SHAREHOLDER PROPOSALS

         Any proposals of shareholders intended to be presented at the 2000
Annual Meeting of Shareholders must be received by the Secretary of the Company
at its principal executive offices at W239 N1700 Busse Road, P.O. Box 190,
Pewaukee, Wisconsin 53072, on or before December 8, 1999, to be considered for
inclusion in the Company's Proxy Statement and proxy relating to such meeting.
In order to be presented at the 2000 Annual Meeting Shareholders, such proposals
must also comply with Article II, Section 2.15, of the Company's Bylaws.




                                        7

<PAGE>   10


                                  OTHER MATTERS

         The Board of Directors is not aware of any business to come before the
meeting other than those matters described above in this Proxy Statement.
However, if any other matter should properly come before the meeting, holders of
the proxies will act in accordance with their best judgment.

         The cost of solicitation of proxies will be borne by the Company. The
Company will reimburse brokerage firms and other custodians, nominees and
fiduciaries for reasonable expenses incurred by them in sending proxy materials
to the beneficial owners of Common Stock. In addition to solicitation by mail,
directors, officers and regular employees of the Company and/or the Bank may
solicit proxies personally or by telegraph or telephone without additional
compensation.

                                          BY ORDER OF THE BOARD OF DIRECTORS



                                          George R. Schonath
                                          President and Chief Executive Officer

Pewaukee, Wisconsin
April 6, 1999

                                        8