Cytogenetic studies performed on 130 consecutive childhood acute nonlymphocytic leukemias (ANLL) investigated in the same center between 1977 and 1986 are reported. The incidence of clonal chromosomal abnormalities was 68.5% with uneven distribution among the groups of the FAB nomenclature. The high incidence of t(8;21) translocation cases, which was 58.6% of M2 ANLL cases, was remarkable. Complete remission rate was lower (P less than 0.05) in ANLL with all karyotypically abnormal metaphases (AA) than in the other ANLL (NN, with only normal metaphases, and AN, with a mixture of normal and abnormal metaphases). Median survival was also shorter in AA ANLL than in AN and NN cases (P less than 0.01). Median survival was different according to karyotype abnormalities: 11q anomalies and t(15;17) were not associated with a good prognosis, and the t(8;21) is not associated with a particularly long median survival (16 months) when compared with other ANLL as opposed to the results of others. The longest survival (26 months) was observed in patients with acute myelomonocytic leukemia with bone marrow eosinophilia. It may be concluded that chromosome studies have a prognostic value in childhood ANLL.