Seventy-six patients, aged 2 to 17 years, were treated with bone marrow transplantation for severe aplastic anaemia or acute leukaemia refractory to conventional therapy. 16 of the 22 patients (73%) who received marrow transplantations for aplastic anaemia are surviving, 12 of these for over one year. In acute leukaemia, using preparation with cyclophosphamide and total body irradiation, 8 of 33 patients (24%) receiving allogeneic and 5 of 8 (63%) receiving syngeneic transplantations are continuing in remission from 3 months to beyond 2 years. The longest continuing remission off therapy is now over 4 1/2 years after preparation with total body irradiation. The major causes of failure remain graft-versus-host disease, infection, graft rejection (aplastic anaemia), and leukaemic relapse.