We analysed forty consecutive patients with acute nonlymphocytic leukemia (ANLL) using methotrexate cell synchronization and 24 h-unstimulated cultures of bone marrow cells to determine the incidence of chromosomal aberrations and the association of specific anomalies with FAB morphological subtypes, in an Indian population. All patients demonstrated an abnormal karyotypic pattern. The specific chromosomal changes viz., t(9;22), t(8;21), t(15;17), t/del(11q), 12p- were found in M 1(3/5), M2(8/15), M3(8/8), M4(1/1) M5(2/4) and M2Ba(1/1) (M2 with Basophilia) patients. Abnormalities of 11q were also noted in two M2 patients showing monocytic involvement. A translocation involving chromosomes 6 and 9 was seen in one patient with M1 and two patients with M2. An inv(16) was observed in M1 (one case), M2 (two cases) and M6 (one case). A del(16) was noted in an M4 case. Although t(9;22) is frequently associated with M1 patients, it was also detected in M2 (two patients) and M4 (one patient). Among all the FAB specific anomalies described above, t(8;21) and t(15;17) were observed only in M2 and M3 patients, respectively. Interestingly, one M2 patient had two independent clones, one with t(8;21) and t(9;11). Deletion or translocation involving 11q was found in a Ph positive M4 patient. New structural rearrangements such as t(1;7) (q32;q36) in association with t(8;21), and t(14;22) (q32;q11) in association with del(11)(q23) were detected in a M2 and a M5 patient, respectively. In conclusion, our studies have revealed that the incidence of FAB specific abnormalities viz., t(8;21); t(15;17), t(9;22), t/del(11q) and also other recurrent anomalies viz., -7/7q-, +8 is much higher in our patients, as compared with other countries. This difference may be attributed to the influence of differential environmental exposure to unknown carcinogenic agents.