Allogeneic marrow transplantation using related marrow donors for myelodysplasia (MDS) and acute myeloid leukaemia (AML) arising from MDS results in 35-56% actuarial disease-free survival. Because the use of unrelated donors has not been well-characterized, we report on the outcome of 52 patients with MDS or MDS-related AML consecutively treated between 1987 and 1993 with unrelated donor marrow transplantation. The median age was 33 (range 1-53) years. 33 patients received chemotherapy and total body irradiation and the remainder busulfan and cyclophosphamide. The donors were phenotypically identical at the HLA-A, B and Dw/DRB1 loci in 34 cases and mismatched for one HLA locus in 17 cases and two loci in one case. Marrow was non-T-cell depleted and methotrexate with cyclosporine or FK506 was used for postgrafting immunosuppression. The 2-year disease-free survival, relapse, and non-relapse mortality rates were 38%, 28% and 48%, respectively. One patient who relapsed survives disease-free after withdrawal of immunosuppressive therapy. 16/19 survivors have a performance status of 90-100%. Patients with MDS in transformation or with AML had a significantly higher risk of relapse than patients with less advanced disease (P = 0.0014). Increased non-relapse mortality was significantly associated with higher age, longer disease duration before transplant, lower neutrophil count on admission and, unexpectedly, being seronegative for cytomegalovirus. We conclude that the outcome with transplantation using unrelated donors is similar to reported results using related donors and that a meaningful proportion of eligible patients with an otherwise incurable disease may be cured with this treatment. However, mortality from the transplant procedure is high and future studies should focus on reducing toxicity.