Bortezomib therapy has become an important part of the standard of care for patients with relapsed multiple myeloma, and preliminary clinical evidence suggests that bortezomib retreatment in patients previously treated with the drug may prolong disease control. This retrospective study was designed to clarify the utility of bortezomib as a repeat therapy. We reviewed records from 3 major cancer centers that had participated in the phase II (SUMMIT or CREST) or phase III (APEX) registration studies to identify patients who were subsequently retreated off protocol with bortezomib-based therapy. We found 22 patients who received bortezomib retreatment following a 60 or more day gap between bortezomib treatments. Twelve patients had intervening therapy between initial bortezomib treatment and bortezomib retreatment. During retreatment, 14 of 22 patients received bortezomib in combination with another antineoplastic agent. The overall response rate for bortezomib retreatment was 50% (9% complete responses). The median length of retreatment was 5.1 months in responding patients and 2.4 months in nonresponding patients. Therapy was terminated due to unmanageable toxicity in 2 patients during retreatment, compared with 6 patients during initial treatment. During retreatment, no patients required dose reduction due to peripheral neuropathy, compared to 4 patients during their initial treatment. Thus, bortezomib retreatment appears to be safe and effective. Favorable observed response rates with bortezomib retreatment suggest that it may be a viable option for relapsed or refractory multiple myeloma, even in patients previously exposed to bortezomib.