The flow cytometric immunoassay was used to study the correlation between the H-ras oncogene product p21 and the DNA ploidy in 30 de novo cases of acute myelogenous leukemia (AML). The results showed that 17 cases were negative for p21 expression and 13 positive for p21. The patients with positive p21 had higher percentage of bone marrow and peripheral blasts and lower peripheral leukocyte count. The expression of p21 had no influence on the therapeutic effect. Before treatment, DNA diploidy occurred in 18 cases including 13 p21 negative ones, and DNA aneuploidy was revealed in 12 cases including 8 p21 positive ones. Patients with positive p21 or having aneuploidy in complete remission were at risk for early relapse. Our results suggest that p21 may be involved in the process of leukemogenesis and progression in AML.