Graft-versus-host disease (GVHD) remains a major cause of morbidity and mortality after haematopoietic stem cell transplantation from matched unrelated donors (MUD). The role of T-cell depletion (TCD) as a strategy to prevent GVHD is controversial because of the associated increased risk of leukaemic relapse, graft failure and delayed immune reconstitution. The demonstration that donor lymphocyte infusion (DLI) is effective salvage therapy if patients relapse after transplantation for chronic myeloid leukaemia (CML) prompted us to examine the proposal that TCD may be a form of GVHD prophylaxis particularly suited to this disease in patients undergoing MUD transplantation. We analysed the outcome of 106 consecutive patients with CML in first chronic phase who underwent MUD transplantation. Patients were conditioned with cyclophosphamide and total body irradiation (TBI), and received in vivo TCD, using CD52 monoclonal antibody, as GVHD prophylaxis. Donor lymphocytes were infused at the time of leukaemic relapse. The projected survival at 5 years for all patients was 52·6%. The probability of developing severe acute GVHD (grade 3 or 4) was 14·5%. The only significant predictor of overall survival in univariate and multivariate analysis was patient cytomegalovirus (CMV) serostatus: in CMV-negative patients survival at 5 years was 60% vs. 42% in CMV-positive patients (P = 0·006). The use of TCD was associated with an increased risk of relapse (62% probability at 5 years after transplant), but 80% of patients who received DLI achieved molecular remission that was durable in all but two cases. In vivo TCD, in conjunction with DLI at relapse, is a valuable GVHD prophylactic regimen in CMV-seronegative recipients of MUD allografts, but in CMV-seropositive patients this approach is associated with an increased non-relapse mortality. Consequently, GVHD prophylactic regimens in MUD transplantation should be tailored according to the patient and donor pretransplant characteristics.