Abstract Purpose: We did a randomized phase I/II trial designed to evaluate the safety and efficacy of combining the proteasome inhibitor bortezomib with high-dose melphalan as the conditioning for high-dose therapy and autologous transplant for myeloma. Experimental Design: Enrolled patients were limited to those who did not achieve a very good partial remission (VGPR) following one or more induction regimens, and were randomized to receive a single escalating dose of bortezomib (1.0, 1.3, or 1.6 mg/m2) either 24 hours before or 24 hours after high-dose melphalan. Dose escalation was based on the escalation with overdose control (EWOC), a Bayesian statistical model. Bone marrow aspirates were collected before initiation of therapy and at the time of transplant to evaluate which sequence resulted in maximal plasma cell apoptosis, and response to transplant was assessed by the International Myeloma Working Group criteria. Results: Among 39 randomized patients, 20 received bortezomib after melphalan and 19 received bortezomib before melphalan. Toxicities and posttransplant hematopoietic recovery rates were similar between arms. The overall response rate for all patients was 87%, with 51% achieving a VGPR or better. Pharmacodynamic studies showed greater plasma cell apoptosis among patients who received bortezomib following melphalan. Conclusions: The use of bortezomib in conjunction with high-dose melphalan is safe, with data suggesting improved efficacy. A single dose of bortezomib administered after high-dose melphalan is the recommended dose and schedule for future clinical investigation. Clin Cancer Res; 16(20); 5079–86. ©2010 AACR.