Complete data regarding age, sex, karyotype, and French-American-British Cooperative Group classification were available for 239 unselected patients with acute nonlymphocytic leukemia. Of these, 128 were classified as having acute myeloblastic leukemia (M1 or M2); within the acute myeloblastic leukemia group, 83 (65%) of the patients were chromosomally abnormal. Except for 16 patients with a t(8;21), the percentage of patients with an abnormal karyotype increased with age, particularly above the age of 50 years. Besides the patients with t(8;21), there were 29 patients with loss of part or all of chromosomes 5 and/or 7, 11 patients with +8, and 27 patients with other abnormalities. Of 70 patients with acute myelomonocytic leukemia (M4), on the other hand, 28 (40%) were chromosomally abnormal, only three had loss of chromosomes 5 or 7, one was -7, +8, and our were +8, whereas 20 had other abnormalities. This difference may reflect different etiological factors in these two types of leukemia.