Sixty three patients aged 27–66 years (median 52) were allografted from HLA-matched sibling (n= 47), 10 of 10 allele-matched unrelated (n= 19), or one-antigen/allele-mismatched (n= 7) donors aged 24–69 years (median 46) after a conditioning regimen comprising 100 mg/m 2 melphalan. Cyclophosphamide (50 mg/kg) was also administered to patients who had not been autografted previously. Cyclosporine or tacrolimus, and mycophenolate mofetil were administered to prevent graft-versus-host disease (GVHD). The 2-year cumulative incidences of relapse and TRM were 55 and 24% respectively, and 2-year probabilities of overall survival (OS) and disease-free survival (DFS) were 36 and 21%, respectively. Poor performance status, donor age> 45 years and elevated lactate dehydrogenase (LDH) increased the risk of treatment-related mortality (TRM), refractory disease and donor age> 45 years increased the risk of relapse, and OS and DFS were adversely influenced by refractory disease, poor performance status, increased LDH, and donor age> 45 years. Our data suggest that younger donor age is associated with better outcome after sub-myeloablative allogeneic hematopoietic stem cell transplantation (HSCT) for hematologic malignancies due to lower TRM and relapse. This finding raises the question of whether a young 10-allele-matched unrelated donor is superior to an older matched sibling donor in patients where the clinical situation permits a choice between such donors.