Cytogenetic studies were performed on 32 consecutive Chinese patients with de novo acute nonlymphocytic leukemia (ANLL) in Taiwan. Of the 30 patients with adequate specimens, 20(66%) had clonal chromosome abnormalities. Structural rearrangements were detected in 18 of them. Seven (four were children) of the 16 patients with M2 ANLL had t(8;21). All six patients with acute promyelocytic leukemia (APL; M3 subtype) had t(15;17). Two patients with M4 type leukemia and abnormal bone marrow eosinophils had inv(16)(p13q22). Another M4 patient with a mild increase of morphologically normal eosinophils in the bone marrow had an abnormal chromosome #16, t(1;16)(q21;p13) in which 16q22 was not involved. One patient with M5 ANLL had t(9;11). Only two patients had a numerical change as the sole abnormality. None of the patients had loss or deletion of chromosome #5 or loss of chromosome #7, and only one had a deletion of 7q. This study revealed a high incidence of t(8;21), t(15;17), and a low incidence of -5/5q- or -7/7q- in Chinese patients with ANLL.