Among 42 consecutive recipients of unrelated marrow were 39 HLA-A, -B, -DR identical, matched unrelated donors (MUD) and three with one HLA antigen mismatch. The majority were genomically typed for DRB, DQA, DQB and DPB. The recipients of MUD marrow were compared with 39 recipients of marrow from HLA-identical siblings with similar diagnoses, disease status and age. Each group included 24 patients with hematological malignancies, 6 with severe aplastic anemia and 9 inherited disorders. Immunosuppression consisted of anti-thymocyte globulin (ATG; pre-BMT mainly to recipients of unrelated marrow), CsA and four doses of MTX. Grade I acute GVHD was treated with prednisolone 2 mg/kg. In a comparison of MUD marrow recipients and HLA-identical siblings 34 of 39 and 36 of 39 of the patients engrafted, respectively. Recipients of MUD marrow and HLA-identical siblings achieved 0.2 x 10(9) WBC/l on day 16 (median) and 14, respectively (P = 0.03). Furthermore, the recipients of MUD marrow needed more platelet transfusions (P = 0.04). The incidence of acute GVHD grade II-III was 15% in the MUD marrow recipients compared with 11% among the HLA-identical siblings. The 2-4 year cumulative incidence of chronic GVHD was 29% and 22% in the two groups, respectively. The overall 2-year survival was 59 and 78%, respectively. Among patients with CML in chronic phase or accelerated phase (n = 26), 2-year relapse-free survival was 79% in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)