We report on the leukemic cell karyotype of 180 children with acute myeloblastic leukemia (AML). They were treated by the protocols of chemotherapy for the Children's Cancer and Leukemia Study Group (CCLSG) in the last decade. Of 132 cases with adequate banding analysis, 24.2% had normal karyotype, 21.2% had miscellaneous clonal abnormalities and 54.6% were classified into known cytogenetic subgroups: t(8;21) (n = 35), t(15;17) (n = 23), inv (16) (n = 6), t(11q23;V) (n = 6), -7/7q-(n = 2). Each karyotype was closely correlated with a particular FAB subtype such as t(8;21) in M2, t(15;17) in M3, inv (16) in M4, t(11q23;V) in M5. In the M1+M2 group, although patients with t(8;21) had favorable clinical features such as low WBC counts and less frequent lymphadenopathy, their treatment outcome was not significantly better than those of patients with a normal karyotype (3-year EFS: 58 +/- 11% vs. 47 +/- 12%). Patients with miscellaneous chromosomal abnormalities had a significantly shorter EFS (22% +/- 10%) (p < 0.05) than those with t(8;21) or normal karyotype. In M4+M5 group, 2-year EFS of patients with inv (16) (40 + 30%) was longer than that of patients with normal karyotype (25 +/- 19%), and t(11q23;V) or miscellaneous chromosomal abnormalities (0 +/- 25%). These results suggest that cytogenetic data may be useful for risk-based treatment assignments for children with AML.