We studied the karyotype in 26 children with ANLL, which was diagnosed on the basis of the FAB classification. Clonal chromosome abnormalities were found in 21 of 26 patients. Four patients, including 3 with Down's syndrome, had AML(M1). Nine patients, including 3 with t(8;21), had AML(M2). All 3 patients with APL(M3) had t(15;17). Four patients had AMMOL(M4); 3 of these had a normal karyotype. Six patients had AMOL(M5); 5 and 11q rearrangements, and 3 of these had a break in 11q23. Only one patient had EL(M6), and he had a normal karyotype. One patient with t(11;19), classified as AML(M2) on Wright-Giemsa-stained cells, had a strong alpha-naphthyl acetate esterase reaction, indicating that the leukemic cells had a cytochemical feature characteristic of monocytes. Whereas t(8;21) and t(15;17) are uniquely associated with AML(M2) and APL(M3), respectively, the 11q rearrangements are also seen in AML(M1/M2), although they are more common in AMOL(M5) and AMMOL(M4). The case with t(11;19) suggests that cells with 11q rearrangements and with AML(M1/M2) may have both monocytic and granulocytic features. When we used our data and previous reports on 243 aneuploid patients (169 adults and 74 children) to correlate the chromosome abnormalities with patient age, we found differences in the chromosome pattern seen among various age groups. This suggests that different etiologic factors as well as changes in host susceptibility may influence the development of and the karyotypic pattern in the various types of leukemia. Moreover, the frequency of various chromosome abnormalities in childhood ANLL can provide a baseline for comparison of the frequency of the same abnormality in adults. The karyotypic analysis of childhood ANLL is important not only because of the information that can be obtained about childhood ANLL, but also because the data can provide substantial insight into the etiology of ANLL in adults.