Seventy-four adult patients with acute nonlymphocytic leukemia (ANLL) were classified retrospectively as to whether or not they had had occupational exposure to insecticides, chemicals and solvents, or petroleum products. Fifty-eight patients were considered nonexposed and 16 were considered exposed. The chromosome banding pattern was abnormal in 37 of the 74 patients (50%). Twenty-five of the 58 (43%) nonexposed patients had a clonal chromosome abnormality, compared with 12 of the 16 (75%) exposed patients (p = 0.02). Only 2 of 23 (8.7%) females with an abnormal karyotype were exposed, whereas 10 of 14 (71%) males with an abnormal karyotype were exposed. Either a -5/5q- or a -7/7q- was present in 67% of the exposed patients with a chromosome abnormality, compared to 28% of the aneuploid nonexposed patients. The -7/7q- abnormality was present in 7 of the 12 (58.3%) exposed patients, versus 5 of the 25 (20%) nonexposed patients with abnormal karyotypes (p less than 0.05). The -5/5q- anomaly was observed in 4 of the 12 (33%) exposed patients and in 4 of the 25 (16%) nonexposed abnormal patients. Our study supports the observation that a subset of patients with ANLL de novo have a history of occupational exposure and a unique pattern of chromosome abnormalities.