BACKGROUND: How tandem autologous-allogeneic stem cell transplantation should be integrated in the treatment of multiple myeloma remains controversial. We examined the long-term outcome of patients with multiple myeloma managed with tandem autologous-allogeneic stem cell transplantation and present a prognostic factor analysis based on the experience of the Société Française de Greffe de Moelle et de Thérapie Cellulaire (SFGM-TC). DESIGN AND METHODS: This French, retrospective, registry-based study included 146 patients who had undergone tandem autologous-allogeneic transplantation for multiple myeloma at 20 SFGM-TC centers between 1998 and 2010. The patients included in the study had fully completed the two steps of a planned tandem autologous-allogeneic transplantation. No treatment had to be administered between the autologous and allogeneic parts of the tandem procedure. RESULTS: Seventy-seven patients (53%) underwent tandem autologous-allogeneic transplantation as part of upfront treatment, i.e. after a single line of treatment not including autologous transplantation. The median follow-up from the allogeneic transplant was 47.5 months (range, 1.2-132 months). At 4 years, the overall survival and event-free survival rates were 48% (95% CI 39-57 %) and 27% (95% CI 19-36), respectively. Eighteen patients (12%) experienced grade III-IV acute graft-versus-host disease and 43 patients (30%) had chronic graft-versus-host disease. The transplant-related mortality rate at 1 year was 15% (95% CI 10-22). Patients receiving tandem transplantation as upfront treatment had significantly improved event-free survival (36% versus 11%; P=0.005) and overall survival (56% versus 34%; P=0.02). Donor's age ≤ 50 years was associated with improved event-free survival (35% versus 16%; P=0.005) and overall survival (54% versus 41%; P=0.02). In the multivariable analysis, upfront tandem transplantation, donor's age ≤ 50 years and full chimerism were independent prognostic factors for better outcome. CONCLUSIONS: We confirmed the feasibility of tandem autologour-allogeneic transplantation in heavily treated patients with multiple myeloma. We identified younger donor's age and upfront tandem transplantation as two independent prognostic factors for survival which could be further explored in prospective studies.