Outcome is poor with conventional therapy for relapsed transformed non-Hodgkin's lymphoma (NHL). Autologous SCT has been successfully employed; however the impact of allogeneic SCT has not been well defined. We therefore studied 40 consecutive patients who received allogeneic SCT for relapsed composite and transformed NHL (25 transformed, 8 composite (same site) and 7 discordant (different sites)) with related (n=25) and unrelated donors (n=15) to evaluate long-term outcome. Conditioning was myeloablative in the majority (39 of 40). Of 40 patients, 11 survive with median follow-up of 25 months. Death occurred in similar proportions due to relapsed NHL (n=14) or treatment-related complications (transplant-related mortality, TRM; n=15). The cumulative incidence of TRM was 36% at 3 years and disease relapse was 42% at 5 years. Probability of 2- and 5-year event-free survival is 36 and 23% with overall survival 39 and 23%. Performance of SCT within 1 year of NHL diagnosis predicted improved outcome. Relapse and TRM remain significant problems in this setting, indicating the need for strategies whereby patients at high risk of transformation should be selected for early SCT, ideally before their actual transformation.