This is an interventional, randomized (assignment to a group is happening by chance, like flipping a coin), open-label, parallel-group, event-driven, international, multicenter, Phase 3 study. This study has different phases: a pre-treatment phase, a treatment phase (which consists of an optimized retreatment period followed by a prolonged therapy period, or a standard retreatment period followed by a posttreatment period), and a long-term follow-up phase for survival up to the end of the study. Before the premature stopping of enrollment, the end of the study was event-driven, defined as 1 year after 186 events (an 'event' being defined as disease progression or death). Following the premature stopping of enrollment, effective 20 June 2014, the end of the study is defined as a maximum of 18 months after enrollment of the last patient in the study. After providing written informed consent, patients will be evaluated for eligibility during a 14-day pre-treatment period. The study doctor will carry out tests to see if the patient is suitable for this study, within the two weeks before receipt of the first dose of the study drugs. Once it has been determined that the patient is able to participate, he/she will be randomly assigned to 1 of 2 different bortezomib retreatment schedules. In this first randomization, patients will receive optimized retreatment or standard retreatment in a 2:1 ratio. Group A: optimized retreatment followed by prolonged therapy. Patients will start therapy with retreatment with 6 cycles of bortezomib and dexamethasone (two 21-day cycles followed by four 35-day cycles) followed by a second randomization in a 1:1 ratio to 1 of 2 prolonged therapy schedules with bortezomib alone (Group A1: once weekly for the first 4 weeks in 35-day cycles; or Group A2: once every other week). Group B: standard retreatment with eight 21-day bortezomib and dexamethasone cycles, followed by posttreatment follow-up every 6 weeks. During the retreatment phase, a series of tests will be done at the first day of each cycle. For patients in Group A the doctor will assess if the patient has responded to the treatment or not. Only if the patient responded, will he/she be able to continue in the prolonged therapy part of the study. At this time the patient will be randomized to one of the two groups (Group A1 or Group A2) in the prolonged therapy phase. During the prolonged therapy phase the disease status and the response to therapy will be evaluated every 6 weeks. This phase will continue until the disease progresses, in case there are unacceptable toxicities despite dose modifications. During the posttreatment period patients in group B will continue to be evaluated for disease status every 6 weeks until confirmed disease progression, when they start alternative multiple myeloma treatment, are withdrawn from the study, death or at the end of the study (a maximum of 18 months after the last patient is enrolled in the study), whichever occurs first. In case the patient discontinues bortezomib before disease progression during the treatment phase, he/she will be asked to complete the End of Study Visit procedures and will be evaluated every 6 weeks until confirmed disease progression, when they start alternative multiple myeloma treatment, are withdrawn from the study, death or at the end of the study (a maximum of 18 months after the last patient is enrolled in the study), whichever occurs first. All patients will have an End of Trial Visit performed 30 to 35 days after the last administration of bortezomib, or as soon as possible after bortezomib treatment is discontinued for patients receiving alternative multiple myeloma therapy. After confirmed disease progression or start of the first alternative multiple myeloma therapy, patients will enter the long-term follow-up phase for up to a maximum of 18 months after the last patient is enrolled in the study. During this phase, the patients will be contacted by at least a telephone call every other month to be followed up for the first alternative multiple myeloma therapy and survival. From the end of the study in countries where bortezomib is not commercially available for prolonged therapy or is not accessible (via a national program or access program) at that time, patients who in the opinion of the investigator would continue to benefit from prolonged therapy with bortezomib, will continue to be supplied with bortezomib until it is accessible in that particular country or for a period of 2 years, whichever occurs first. Before the premature stopping of enrollment, it was planned to enroll a target of 240 patients in this study.