SummaryA bone marrow transplantation was performed in 30 patients because of leukaemia or severe aplastic anaemia. The donor was a monozygous twin in 2 cases, a HLA-matched sibling in 26 and a mismatched sibling in 2 cases. Sixteen patients were transplanted early during their disease (= good prognosis patients); results in these patients are excellent with a projected two-year survival of 81 %, using the Kaplan-Meier method. In this group we observed only one relapse of leukaemia (syngeneic transplant) and two transplantation-related deaths. In contrast, results obtained in 14 patients transplanted in advanced stages of their disease (= poor prognosis cases) are disappointing with a high relapse frequency (5 patients) and an impressive percentage of non-Ieukaemic deaths (6 patients). Only 3/14 are alive and well and in first complete remission after transplantation. Bone marrow transplantation, performed early during the disease offers an excellent chance of prolonged survival and potential cure.