Seventy-three patients with acute nonlymphocytic leukemia in first complete remission (CR) have received allogeneic bone marrow transplantation (BMT) with non-T-lymphocyte-depleted marrow obtained from matched sibling donors. The first 36 patients received a preparative regimen consisting of cyclophosphamide, 60 mg/kg/d (days -6 and -5), and 750 cGy single-dose total-body irradiation (TBI) (day -1). Subsequently, 37 patients received cyclophosphamide 60 mg/kg/d (days -6 and -5), and 165 cGy fractionated TBI administered twice daily for a total dose of 1,320 cGy (days -4, -3, -2, and -1). Survivors have been followed from 9 to 124 months (median, 40 months). The 61% (95% confidence interval [CI], 45% to 77%) projected disease-free survival (DFS) of 41 children less than 18 years old does not differ significantly from the 62% (95% CI, 49% to 73%) projected DFS of 32 adults at 84 months (P = .89). Similarly, the 15% (95% CI, 1% to 29%) projected relapse rate seen in children does not differ from the 9% (95% CI, 0% to 21%) seen in adults (P = .69). Multivariate Cox regression analysis of presenting features demonstrates that a presenting WBC count greater than 20,000/m3 is associated with decreased DFS (P = .01). When compared with other French-American-British (FAB) subtypes, presentation with FAB M4 or M5 morphology is significantly associated with relapse in multivariate analysis (P = .014). Other presenting features such as preparation with single-dose or fractionated TBI, interval from diagnosis to CR or CR to BMT, donor or recipient sex, and donor or recipient cytomegalovirus serology do not correlate independently with either DFS or relapse. When included in the stepwise multivariate analysis of presenting patient features, two posttransplant events, development of grades 2 to 4 acute graft-v-host disease (GVHD) (P less than .03) and development of interstitial pneumonitis (P less than .001), also correlate independently with poor DFS. Allogeneic BMT provides equivalent, prolonged DFS in both children and young adults when performed in first CR and should be considered the therapy of choice for all first CR patients under 45 years of age with a suitable donor. Continued efforts to prevent and treat acute GVHD and pneumonitis as well as efforts designed to prevent relapse in patients presenting with FAB M4 and M5 morphology should further improve outcome.