Bone marrow (BM) karyotypes from 16 consecutive children presenting with nonlymphocytic leukemia were established with the use of banding techniques, before therapy. The two patients with chronic myeloid leukemia (CML) showed the Philadelphia (Ph1) translocation (9q+;22q-). Five of the 14 patients with an acute nonlymphocytic leukemia (ANLL) presented no acquired cytogenetic abnormalities, but one of these five showed a high level of hypodiploidy. One patient with AML evidenced a variant of the Ph1 chromosome originated as a translocation (12p+;22q-). Nonrandom abnormalities (-7; 7q-; +8; t(8;21); -21) were found in six patients, isolated or in association with otheraberrations. Among the random abnormalities, apparently balanced translocations and chromosomal deletions were observed. In ANLL, no correlation could be found between morphologic diagnosis and cytogenetic findings. On the other hand, the presence of BM cells with a normal karyotype at diagnosis was associated with an improved remission rate and survival time. Followup studies were performed in four ANLL patients with an abnormal cell clone at diagnosis. Three of them achieved hematologic remission; their BM karyotype was found to be normal at that stage. In the 4th patient, generalization of the abnormal karyotype in BM cells was seen in the terminal phase of the disease.