During the last 10 years, we have cytogenetically analyzed at diagnosis bone marrow cells from a total of 94 children with acute leukemia. Of the 78 children with acute lymphatic leukemia (ALL), 53 (68%) had clonal acquired chromosome abnormalities; in the group with acute nonlymphatic leukemia (ANLL), the corresponding proportion was 13 out of 16 (81%). Among the cytogenetically abnormal ALL patients, the most numerous subset was the hyperdiploid cases with stemlines containing 51 or more chromosomes (26 of 53 abnormal cases; 49%). This is a clearly higher proportion than has been reported in large series from other centers. Deletions of 6q were present in 8 cases and rearrangements of 12p in 5. Of the 7 T-cell ALLs, 3 had translocations of the distal part of 7q, i.e., of the region where the beta T-cell receptor is encoded. Only 2 of 26 (8%) patients with leukemic stemlines with more than 50 chromosomes have relapsed; the remainder are still in first remission (mean observation time 42 months). This may be contrasted with 6 of 25 (24%) relapses among the cytogenetically normal (observation time 41 months), and 8 of 27 (30%) relapses among ALL patients with aberrations but with less than 51 chromosomes (observation time 26 months). Our results support the conclusion that the finding of a markedly hyperdiploid leukemia karyotype is indicative of good prognosis in ALL.