Thirty-six children with acute lymphoblastic leukaemia (ALL) in second remission were treated with conventional chemotherapy or with cyclophosphamide and fractionated total-body irradiation followed by an allogeneic bone marrow transplant; the choice of treatment was dictated by the availability of an HLA-identical sibling. The age, sex, clinical data at presentation of the disease and duration of first remission were comparable for the two groups of patients. In the bone marrow transplantation group two patients died of graft-versus-host disease and five of leukaemia. Ten patients survive, nine disease free, 13-53 months from second remission (6-51 months post-bone marrow transplantation). In the chemotherapy group 14 patients died of leukaemia (2-29 months from second remission) and five survive (22-34 months from second remission). The actuarial survival for patients with bone marrow transplantation is 48% at 4 years as compared with 22% for those of the chemotherapy group (P = 0.04); the actuarial probabilities of being in remission are 58 and 18% in the two groups respectively (P = 0.01). This study confirms that allogeneic bone marrow transplantation is superior to chemotherapy in patients in second remission with ALL and should be considered in the presence of an HLA-identical sibling.