Analyses of the cellular DNA content were carried out by flow cytometry (FCM) in 148 adult patients with acute myeloid leukemia (AML) to assess the incidence of DNA aneuploidies and its relation to patient characteristics and morphologic subtypes. DNA aneuploidies were found in 54 of 131 patients with de novo AML (41.2%) and in 4 of 17 patients with AML after preleukemic syndromes. Subclassification according to morphology revealed the lowest rate of DNA aneuploidies in M1 leukemias (25%) and a significantly lower degree of DNA aneuploidy in M1 and M2 cases as compared to M4 and M5 subtypes (P less than 0.05). Within the group of M4 and M5 leukemias, patients less than or equal to 40 years of age had a higher frequency of aneuploid DNA stemlines (71.4%) than older patients (33.3%) (P less than 0.025). No differences between patients with and without DNA aneuploidy were identified for the initial leukocyte count, serum LDH, bone marrow S-phase index, bone marrow cell count/mm3 bone marrow nor the initial response to the induction regimen of 6-thioguanine, cytosine arabinoside, and daunorubicin (TAD). For remission duration a tendency towards a higher proportion of lung remissions was observed in patients with DNA aneuploidy.