The outcome of 29 multiple myeloma patients receiving fludarabine and melphalan-based non-myeloablative allogeneic transplant (NMT) was evaluated. Event-free survival (EFS) at 24 months was 33%, being significantly higher for patients who developed chronic graft-versus-host disease (cGVHD) when compared with those who did not [51%vs 0% respectively, P = 0.02; hazard rate = 3.16 (95% confidence interval = 1.09-9.15, P = 0.03)] as well as for patients transplanted in complete remission/partial response (CR/PR) or stable disease (SD), compared with those with refractory/progressive disease (43%vs 0% respectively, P = 0.02). Overall survival (OS) at 24 months was 60%[72%vs 42% for patients who did and did not develop cGVHD respectively (P = 0.1); 63%vs 41% for patients in CR/PR or SD vs refractory/progressive disease at transplant respectively (P = 0.013)]. At a median follow-up of 366 d, 13 patients remained in CR/PR (45% overall response rate). Nine patients have died, three of them as a result of disease progression and six (21%) as a result of transplant-related mortality (TRM). Actuarial incidence of TRM was 37% for patients who developed acute GVHD vs 13% for those who did not (log rank, P = 0.04). The present study suggests that graft-versus-myeloma effect is the main weapon for disease control after NMT in MM patients and the efficacy of this immune effect depends on tumour burden before transplant.