Leukemia karyotypes were analyzed in 792 children with acute lymphoblastic leukemia (ALL) and 217 patients with acute myelocytic leukemia (AML). These patients were registered and uniformly treated in German multicentre trials from 1984-01-01 to 1989-12-31. In distinct leukemia subgroups specific chromosome abnormalities were found: Numerical aberrations such as hyperdiploidy over 50 chromosomes in c-ALL or structural aberrations (translocations) such as t(8;14) in B-ALL, t(11;14) in T-ALL, t(4;11) in ppB-ALL, t(1;19) in pB-ALL, t(15;17) in AML-M3, t(8;21) in AML-M2. Prognostic significance of the leukemia karyotype probably can be changed by intensive cytotoxic chemotherapy. Unfavorable prognosis, however, still persists in t(9;22) and t(4;11); "favorable" prognosis can be seen in t(8;21) and t(15;17). Inherited or induced chromosome instability is discussed as a possible predisposing factor for the origin of chromosome aberrations.