In order to investigate the relationship between aberrant marker expression and DNA ploidy, 61 adult patients with acute leukemia (39 AML and 22 ALL) were studied. Aberrant marker expression was observed in 20 patients (16/39 of AML and 4/22 of ALL patients). In flow cytometric DNA analysis aneuploidy was observed in 18 patients (9/39 of AML and 9/22 of ALL patients). The incidence of aneuploidy in patients with aberrant marker expression was 35% whereas this was 26.8% in patients without aberrant marker expression. Futhermore, 7 patients with aberrant marker expression showed an aneuploid, double G(0)/G(1) peaks appearance whereas the remaining 11 patients with aberrant marker expression had euploid DNA content. Double G(0)/G(1) appearance was not observed in patients without aberrant marker expression. Further analyses revealed that this did not correlate with apoptosis. All 7 patients, who had both aberrant marker expression and double G(0)/G(1) peal; had a poor clinical outcome with a short survival and all died within three months whereas three-months survival was 67% for AML, 69% for ALL patients and 81% for patients with aberrant marker expression respectively (p<0.01). Our data indicate that the evaluation of the DNA ploidy in patients with aberrant marker expression may be of prognostic importance.