Detailed cytogenetic analyses were performed on specimens from 198 patients with de novo acute nonlymphocytic leukemia (ANLL), including high-resolution banding studies in 79 patients. One hundred ninety-two patients received induction therapy with daunorubicin and cytosine arabinoside (Ara-C) with an overall complete response rate (CR) of 63%. Responding patients received repetitive cycles of Ara-C-based intensification therapy. Clonal abnormalities were detected in 69% of the patients with specimens adequate for cytogenetic analysis. Certain cytogenetic changes were closely associated with French-American- British (FAB) morphology, age, and outcome: t(8;21) (closely associated with FAB M2), t(15;17) (associated with FAB M3), and abn 16q22 (associated with FAB M4EOS) tended to occur in younger patients and were associated with favorable outcomes in terms of both CR rate and long-term disease-free survival. In contrast, 19% of patients who had - 5/5q- and or -7/7q- and seven patients with trisomy 8 were older, had a poor prognosis, and usually failed to achieve remission (CR) because of chemotherapy-resistant leukemia. The adverse effect on CR rate and duration in this group of patients was independent of age, and there was no association with particular morphologic subtypes. These data suggest that cytogenetic findings should influence future therapeutic choices. In particular, patients with abnormalities associated with poor responses may be considered for investigational approaches and may also provide insights into mechanisms of drug resistance.