UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

(Amendment No. 1)

(Mark One)



|x | ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
|- | ------------------------------------------------------------------------------------

For the
fiscal year ended December 31, 2015

or



| ̈ | TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
|- | ----------------------------------------------------------------------------------------

For
the transition period from  to

Commission
File Number: 001-36730

INC RESEARCH HOLDINGS, INC.

(Exact name of registrant as specified in its charter)





|                                                               |  |                                     
|-------------------------------------------------------------- |  | ------------------------------------
|Delaware                                                       |  | 27-3403111                          
|(State or other jurisdiction of incorporation or organization) |  | (I.R.S. Employer Identification No.)
|                                                               | 
|3201 Beechleaf Court, Suite 600
Raleigh, North Carolina        |  | 27604-1547                          
|(Address of principal executive offices)                       |  | (Zip Code)                          

Registrants telephone number, including area code: (919) 876-9300

Securities registered pursuant to Section 12(b) of the Act:



|                                                |  |                                          
|----------------------------------------------- |  | -----------------------------------------
|Title of each class                             |  | Name of each exchange on which registered
|Class A Common Stock, par value $0.01 per share |  | The NASDAQ Stock Market LLC              

Securities registered pursuant to Section 12(g) of the Act: None





Indicate by check mark if the
registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Yes x No  ̈

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or section 15(d)
of the Exchange Act. Yes  ̈ No x

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the
preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes x No  ̈

Indicate by
check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or
for such shorter period that the registrant was required to submit and post such files). Yes x No  ̈

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained
herein, and will not be contained, to the best of registrants knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K. x

Indicate by check mark whether the registrant is a large accelerated
filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See definition of large accelerated filer, accelerated filer and smaller reporting company in Rule 12b-2 of the Exchange Act.
(Check one):



|                        |  |                                                 |  |                           |  |  
|----------------------- |  | ----------------------------------------------- |  | ------------------------- |  | -
|Large accelerated filer |  | x                                               |  | Accelerated filer         |  |  ̈
|                        |  |                                                 | 
|Non-accelerated filer   |  |  ̈ (Do not check if a smaller reporting company) |  | Smaller reporting company |  |  ̈

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act). Yes  ̈ No x

The aggregate market value of the registrants common stock held by non-affiliates of the registrant, based on the closing sale price of on June 30, 2015 (based
on the closing sale price of $40.12 on that date), was approximately $718,968,173. Common stock held by each officer and director and by each person known to the registrant who owned 10% or more of the outstanding common stock have been excluded in
that such persons may be deemed to be affiliates. This determination of affiliate status is not necessarily a conclusive determination for other purposes.

As of
February 18, 2016, there were approximately 53,976,093 shares of the registrants common stock outstanding.

Portions of the registrants Proxy
Statement for its 2016 Annual Meeting of Stockholders are incorporated by reference into Part III hereof.



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EXPLANATORY NOTE

INC Research Holdings, Inc. (the Company) is filing this Amendment No. 1 (Amendment No. 1) to its Annual Report on Form 10-K for the fiscal year
ended December 31, 2015, which was originally filed with the Securities and Exchange Commission on February 25, 2016 (the Original Annual Report), for the sole purpose of correcting a typographical error to reflect the
Companys status as a well-known seasoned issuer, as defined in Rule 405 of the Securities Act of 1933.

Pursuant to Rule 12b-15 under Securities Exchange
Act of 1934, as amended, this Amendment No. 1 on Form 10-K/A also contains new Rule 13a-14(a)/15d-14(a) Certifications, which are attached hereto. Because no financial statements have been amended by or included in this Amendment
No. 1 on Form 10-K/A and this Amendment No. 1 on Form 10-K/A does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4 and 5 of the certifications have been omitted.

This Amendment No. 1 on Form 10-K/A continues to speak as of the date of the filing of the Original Annual Report, and the Company has not changed the financial
statements nor updated the disclosures contained therein to reflect any events that occurred at a later date.



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SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the
undersigned, thereunto duly authorized.



| |  |                             |  |                        |  |                                                                   
| |  | --------------------------- |  | ---------------------- |  | ------------------------------------------------------------------
| |  | INC Research Holdings, Inc.
| |  |                            
| |  | By:                         |  | /s/ D. Jamie Macdonald
| |  |                             |  | Name:                  |  | D. Jamie Macdonald                                                
| |  |                             |  | Title:                 |  | Chief Executive Officer (Principal Executive Officer) and
Director
| |  |                             |  | Date:                  |  | March 14, 2016                                                    



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EXHIBIT INDEX



|               |      |                                                       |                     |                                                                                                                                  |      |   |          |   |         |   |             |               
|-------------- | ---- | ----------------------------------------------------- | ------------------- | -------------------------------------------------------------------------------------------------------------------------------- | ---- | - | -------- | - | ------- | - | ----------- | --------------
|               |      | Incorporated by Reference (UnlessOtherwise Indicated)
|Exhibit Number |      |                                                       | Exhibit Description |                                                                                                                                  | Form |   | File No. |   | Exhibit |   | Filing Date
|               | 31.1 |                                                       |                     | Certification of Chief Executive Officer pursuant to Section 302
of the Sarbanes-Oxley Act of 2002.                              |      | - |          | - |         | - |             | Filed herewith
|               | 31.2 |                                                       |                     | Certification of Executive Vice President and Chief Financial
Officer pursuant to Section 302 of the Sarbanes-Oxley Act of 2002. |      | - |          | - |         | - |             | Filed herewith