Clonal chromosome abnormalities of 343 patients with de novo acute nonlymphocytic leukemia (ANLL) have been tentatively correlated with prognosis. All the patients were treated according to therapeutic protocols in the same hospital. The complete remission rate and median survival were generally lower in AA-ANLL (ANLL with only karyotypically abnormal metaphases) when compared with NN- and AN-ANLL. Similarly, AA-ANLL had the poorest prognosis in the majority of the classes of the French-American-British nomenclature. ANLL with inversion and/or deletion of chromosome #16 had the best prognosis, and ANLL with t(8;21) was not particularly favorable, nor was acute promyelocytic leukemia with t(15;17). ANLL with complex chromosomal abnormalities had the poorest prognosis. The conclusion is that chromosomal aberrations do have a prognostic significance in ANLL, but that this significance is dependent on the types of chromosomal aberrations.