We report results from a study exploring the combination of romidepsin, bortezomib, and dexamethasone for the treatment of patients with multiple myeloma (MM) previously treated with > 1 prior therapy. The primary objective was to determine the maximum tolerated dose (MTD) of the combination using a novel accelerated dose-escalation schedule in patients with relapsed or refractory MM. The secondary objective was to determine overall response (OR), time to progression (TTP), and overall survival (OS). The MTD identified was bortezomib 1.3 mg/m(2) (days 1, 4, 8, and 11), dexamethasone 20 mg (days 1, 2, 4, 5, 8, 9, 11, and 12), and romidepsin 10 mg/m(2) (days 1, 8, and 15) every 28 days. Thrombocytopenia (64%) was the most common ≥ grade 3 hematologic toxicity. Peripheral neuropathy occurred in 76% of patients (n = 19) (≥ grade 3, 8%; 95% confidence interval [CI] 1%-26%). Maintenance romidepsin 10 mg/m(2) (on days 1 and 8 of a 28-day cycle) proved feasible, with 12 patients receiving a median of 7.5 cycles (range: 1-29). An OR (M-protein) of > minor response (MR) was seen in 18 of 25 patients (72%); 2 (8%) had complete remissions (CRs) and 13 (52%) had partial responses (PRs), including 7 (28%) with very good PRs (VGPRs). The median TTP was 7.2 (95% CI: 5.5-19.6) months, and the median OS was > 36 months. This regimen shows activity with manageable toxicity and warrants further evaluation. This trial was registered at www.clinicaltrials.gov as NCT00431990.