The possibility that oxymetholone might induce or enhance leukemia after androgen therapy for aplastic anemia prompted us to study the direct action of oxymetholone on the DNA synthesis of AML cells in vitro. The peripheral blasts of 10 patients, 8 with AML and 2 with CML in blast crisis have been studied. The DNA synthesis of the leukemic cells with and without oxymetholone was measured by the 3H-methyl-thymidine incorporation determined by liquid scintillation. The results have been shown a wide variation of DNA synthesis from patient to patient with a range from 2,000 to 40,000 cpm but no significant difference between test and control cultures. We may conclude that oxymetholone does not increase directly the proliferation capacity of the peripheral AML cells cultured in vitro.