Analyses of the cellular DNA content were carried out in 226 patients with acute lymphoblastic leukemia (ALL) and in 61 children with acute myeloid leukemia (AML) to assess the incidence and clinical significance of DNA aneuploidies. All children were treated within the BFM studies ALL 79/81 and 81/83 as well as AML 78 und 83. DNA aneuploidies were identified in ALL in 39,8% and in AML in 36,1% of cases. Within the ALL group a significantly higher rate of aneuploid DNA stemlines was observed for non-T/non-B ALL with 44,1% as compared to T-ALL with 14,3% (p less than 0,01). In AML the morphologic subgroups M 1/2 revealed a lower frequency of 21,7% DNA aneuploidies than M 4/5 leukemias with 44,4%. The rates of complete remissions were not different between patients with and without DNA aneuploidy neither in ALL nor in AML. In the study ALL 79/81, however, a tendency towards longer remissions for patients with DNA aneuploidy was found (p = 0.053) which could not be confirmed at present by the study ALL 81/83. In both ALL trials patients with the lowest pretherapeutic risk-score revealed the highest rate of aneuploid DNA stemlines.