One hundred fourteen children with acute lymphoblastic leukemia were treated with allogeneic marrow transplantation from HLA-identical siblings after conditioning with cyclophosphamide and total body irradiation. Methotrexate was given posttransplantation for prophylaxis of graft-versus-host disease. The minimum follow-up after transplantation was 2 years. Sixteen of 51 patients transplanted in marrow remission survive from 2.1 to 8.9 years (median 2.7), 13 in continuous remission, one in remission following testicular relapse, and two after marrow relapse. Sixty-three were transplanted in relapse and eight survived 3-10 years (median 5.7), five in continuous remission, and three in remission following testicular relapse. In a multivariate analysis, factors significantly related to increased survival were marrow remission at transplant (p less than 0.007) and chronic graft-versus-host disease (p less than 0.005). Factors associated with increased relapse were marrow relapse at transplant (p less than 0.002) and absence of significant graft-versus-host disease (p less than 0.004). The development of acute graft-versus-host disease was associated with high marrow cell doses (p less than 0.04). These data suggest that some children with acute lymphoblastic leukemia and a poor prognosis with conventional chemotherapy may be cured with marrow transplantation.