Cytogenetic study in 53 children (aged less than 15 years) with acute non-lymphocytic leukemia (ANLL) were studied. The cytogenetic findings were compared with those of ANLL patients (136 aged less than 19 years and 747 aged over 20 years) in the Fourth International Workshop on Chromosomes in Leukemia (IV IWCL) and also with those of childhood acute lymphoblastic leukemia (ALL) cases (previously reported as our 124 ALL case). Of the ANLL patients, 77.4% had acquired chromosomal clonal abnormalities. As abnormalities, t(15;17), all cases which were seen in M3 or M3V cases, t(8;21), which was seen in M1 or M2, and rearrangements of 11q23, which were seen in M5, were more frequently seen than was reported at the IV IWCL (20.8%, 17.0% and 7.5% vs 6.3%, 6.3% and 3.2% respectively). 5q-, monosomy 7, t(6;9) and t(9;22), which have been noted previously in this disease, were not seen. Besides structural abnormalities, some cytogenetic differences in numerical abnormality between ALL and ANLL were observed as follows: 1) Hyperdiploidy of greater than 51 chromosomes noted in ALL was not found in ANLL. 2) Isolated trisomy 8 was frequently found in ANLL, but not in ALL. 3) Loss of a sex chromosome was frequently found in ANLL, but not in ALL. Our study revealed a different frequency of non-random chromosome abnormality in children with ANLL as compared with that of adults, and clarified the differences in numerical abnormalities, as well as structural abnormalities, between ALL and ANLL.