Patients (62) with acute nonlymphoblastic leukemia in 1st relapse or 2nd remission were treated with allogeneic marrow transplantation from HLA-matched siblings. In 17 patients (group 1), no attempt at reinduction of remission was made prior to transplantation. In 20 patients (group 2), attempts at inducing a second remission prior to transplantation were unsuccessful; and in 25 patients (group 3), a second remission was achieved. Five of 17 patients (29%) in group 1, 2 of 20 (10%) in group 2, and 5 of 25 (20%) in group 3 are surviving disease-free 2-6 yr after grafting. Early mortality from nonleukemic causes was equal in the 3 groups, but the risk of recurrent leukemia after transplantation was less in patients transplanted without attempts at reinduction (group 1). Among patients transplanted in relapse, those in early relapse (< 30% blast cells in the marrow) appeared to do better than patients in florid relapse. The results obtained in group 1 are as good as or better than those achieved in patients transplanted in second or subsequent remission. For patients with acute nonlymphoblastic leukemia not transplanted in 1st remission, the optimal time for transplantation would appear to be as soon as possible after the first relapse.