Phase 2 trials have demonstrated that bortezomib +/- dexamethasone is safe and effective in relapsed multiple myeloma (MM). In this multicentre, open-label, phase 3b trial, 638 patients with relapsed or refractory MM (median 3 prior therapies) received bortezomib 1.3 mg/m2 on days 1, 4, 8, and 11 of a maximum of eight 3-week cycles (median 5 cycles). Dexamethasone 20 mg/d was added the day of and day after each bortezomib dose for progressive disease after > or =2 cycles or for stable disease after > or =4 cycles. Responses were assessed based on M-protein changes. Overall response rate was 67%, including 11% complete (100% M-protein reduction), 22% very good partial (75-99% reduction), 18% partial (50-74% reduction), and 16% minimal response (25-49% reduction). Dexamethasone was added in 208 patients (33%), of whom 70 (34%) showed improved response. Median time to best response of minimal response or better was 84 d. Most common grade 3/4 adverse events were thrombocytopenia (39%), neutropenia (16%), anaemia (12%), diarrhoea (7%), and peripheral neuropathy (6%). Neuropathy (any grade) was seen in 25% of the patients and led to discontinuation in 5%. Bortezomib, alone and combined with dexamethasone, is safe and effective in heavily pretreated patients with relapsed or refractory MM.