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 followup 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/microL 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 poor prognostic factors for patients with ANLL who undergo bone marrow transplantation in first remission after conditioning with cyclophosphamide plus total body irradiation.