Chromosome banding patterns were obtained for 50 of 55 consecutive adult patients with acute nonlymphocytic leukemia during a 5-yr period. Twenty-two of the 50 cases were diagnosed as acute myelocytic leukemia (AML), 24 as acute myelomonocytic leukemia (AMMol), 2 as acute promyelocytic leukemia (APL), and 2 as erythroleukemia. Twenty-five patients had initial chromosome abnormalities during the course of the disease. The median survival of patients with normal chromosomes initially (group I) was 10 mo, whereas that of patients with abnormal chromosomes initially (group II) was 2 mo. Similar times were obtained for treated patients with AML and AMMol. However, when the AML patients were separated into those with and those without a chromosome abnormality, the median survival times were markedly different (2 mo versus 18 mo, respectively). Patients with AMMol demonstrated no difference in median survival times when subgrouped according to the presence or absence of chromosome abnormalities. The treated group II patients whose marrow samples had only abnormal metaphases had a poorer response (10% complete remission) and median survival (2 mo) than the group II patients who had at least one normal metaphase (42% complete remission with a median survival of 9 mo). The two cases of APL demonstrated a deletion of the long arm of No. 17 which occurred in the same region of the chromosome in each case. Both patients had similar clinical histories, with disseminated intravascular coagulation, and neither responded to therapy.