From 2002 to 2007, 49 myeloma patients who relapsed following autologous SCT were included in a prospective multicenter trial to determine the efficacy of a reduced melphalan/fludarabine regimen followed by allogeneic SCT from unrelated donors. All patients showed leucocyte and platelet engraftment after a median of 15 and 19 d, respectively. Grade II-IV acute graft-versus-host disease (GvHD) occurred in 25% of patients and 35% had chronic GvHD. Overall response rate at day 100 was 95% including 46% complete remission (CR). Cumulative incidence of non-relapse mortality at 1 year was 25% [95% confidence interval (CI): 13-37%] and was significantly lower for human leucocyte antigen (HLA)-matched compared to -mismatched SCT (10% vs. 53%, P = 0.001). The cumulative incidence of relapse at 3 years was 55% (95% CI: 40-70%). After a median follow up of 43 months, the estimated 5-year progression-free and overall survival rates were 20% and 26% respectively and were significantly better for matched in CR at day 100 (41% vs. 7%, P = 0.04 and 56% vs. 16%, P = 0.02). We conclude that optimal donor selection is mandatory for a low non-relapse mortality and high relapse incidence, which remains a major concern, should be improved by including post-transplant strategies to upgrade remission status.