Prognostic factors were reviewed retrospectively for 39 children and adults aged 1 to 40 years (median 14 years) with acute nonlymphocytic leukemia (ANLL) who attained a first remission and underwent bone marrow transplantation from November 1976 to July 1983. The preparation regimen for transplantation was cyclophosphamide (60 mg/kg/d for two days) followed by total body irradiation (either 750 cGy single dose at 26 cGy/min, n = 37, or 1,320 cGy fractionated at 10 cGy/min, n = 2). Twenty-three patients are surviving disease free with a median follow-up of three years. The three-year estimated disease-free survival is 55% ± 17% (± 2 SE). Five patients have relapsed from 92 to 756 days after transplantation for an estimated relapse rate of 21% ± 18%. Two factors, the white blood cell (WBC) count and the French-American-British (FAB) classification at leukemia diagnosis were found to be of prognostic importance. Patients with a WBC of less than 20,000/μL at diagnosis had a three-year estimated disease-free survival of 74% ± 18% v 26% ± 24% for those with a WBC of greater than or equal to 20,000 (P = .008). The estimated relapse rate was 6% ± 12% for patients with a WBC at diagnosis less than 20,000 v 53% ± 38% for patients with a WBC at diagnosis of greater than or equal to 20,000 (P = .01). Patients with myeloid morphology at diagnosis (FAB M1,2,3) had an estimated relapse rate of 9% ± 12% v patients with monocytoid morphology (FAB M4,5a) whose estimated relapse rate was 58% ± 44% (P = .05). Our data suggest that a high WBC count at diagnosis and monocytoid morphology (FAB M4,5a) are poor prognostic factors for patients with ANLL who undergo bone marrow transplantation in first remission after conditioning with cyclophosphamide plus total body irradiation.