We report here on the leukemic cell karyotypes of 134 children with acute nonlymphocytic leukemia (ANLL) examined at Saitama Children's Medical Center (SCMC), and of 88 children with acute lymphoblastic leukemia (ALL) referred to SCMC. The patients were mainly treated according to the protocol of the Tokyo Children's Cancer Study Group. Of 106 ANLL cases with adequate banding, 18% were normal, 34% had miscellaneous clonal abnormalities, and 48% were classified into known cytogenetic subgroups: t(8;21) (n = 21), 11q23 abnormalities (n = 14), -7/del(7q) (n = 6), inv (16)/del(16) (n = 5), and t(15;17) (n = 5). According to the FAB classification, M7 (21.7%) were more frequent than in previous reports because this study included a number of Down's Syndrome patients with M7 morphology. The present study confirmed the well-known association of t(15;17) with M3, t(8;21) with M2, 11q23 abnormalities with M4 and M5, and inv (16)/del(16) with M4. Patients with t(8;21) or inv (16)/del(16q) ANLL fared no better overall than the entire group. Of 51 ALL cases with adequate banding, 13.7% were normal, and 86.3% were classified into abnormal subgroups: translocation (n = 14), hyperdiploidy (greater than 50) (n = 13), and miscellaneous abnormalities (n = 17). Cases with hyperdiploidy (greater than 50) were restricted to a common phenotype and fared better overall than the entire group. Patients with translocation were found in all phenotypes, and had a poor prognosis. We concluded that childhood acute leukemia could be subgrouped according to karyotypic patterns, and that patients with translocations had a poor prognosis in ALL as well as ANLL.