Treatment outcome and prognostic factors were evaluated in 152 children with acute myeloblastic leukemia (AML) treated on three consecutive protocols (ANLL 861, 8912, 9205) of the Children's Cancer Leukemia Study Group (CCLSG, Japan). In the ANLL 9205 protocol, anthracycline was used with a continuous infusion of cytosine arabinoside, followed by an intensive sequential post remission chemotherapy of short duration. Forty-two of these 46 patients (91.3%) achieved complete remission, and 58.8% of these patients projected a 3-year disease-free survival. These results were apparently superior to those obtained with the ANLL 861 and 8912 protocols, which used conventional doses of multiple drugs followed by a moderate post remission chemotherapy of long duration. This favorable response with the ANLL 9205 protocol was attributed mainly to the high induction rate of patients with the M4 and M5 FAB subtypes, as compared to those in the previous two protocols (93.3% in ANLL 9205 vs. 57.9% in ANLL 861 + 8912; P < 0.05). The ANLL 861 and 8912 protocols, an older age (> or = 8 years), higher WBC counts (> or = 10 x 10(9)/1) and all predicted an increased risk of relapse and decreased the survival following univariate analysis (P < 0.05). An older age and high WBC count continued to predict an increased risk of relapse in multivariate analyses: patients with an age > 8 years and WBC counts > 10 x 10(9)/1 had a 4.5 times higher risk of relapse than patients without these adverse features.