The clinical course of 59 children, who underwent BMT during 1988-1998 with a matched unrelated donor (MUD), was compared with 59 case controls receiving a sibling donor marrow. Thirty-eight patients had haematological malignancies while 21 had a nonmalignant disorder. The cumulative incidence of acute GVHD grade II-IV was 28% for MUD recipients vs 11% (P = 0.014) for sibling recipients. Extensive chronic GVHD was rare in both groups. The 5-year probability of survival was 52% for MUD vs 77% for sibling recipients (P= 0.014). For children with malignancies the 4-year probability of survival was 52% for MUD vs 67% for sibling recipients with a RFS of 49% vs 62%. In the ALL patients the survival of the MUD recipients was 77% and equalled that of the sibling group. For SAA survival was 43% vs 86% (P = 0.09) and for metabolic disorders 63% vs 89% (P = 0.025). The transplant-related mortality was higher in the MUD group, while death due to relapse was equally distributed. These results of MUD BMT in children compare favourably with most previous reports, and support the use of alternative donors in cases who lack an HLA-identical siblings. Bone Marrow Transplantation (2000).