Flow cytometrie analysis of the bone marrowcellular DNA content was performed in 40 adult patients with acute leukemia. Twenty-eight had AML (29 previously untreated and 9 relapse) and 12 ALL (8 previously untreated and 4 relapse). For the whole series, the mean G_1/0-phase compartment was 82±1.3%, the mean S-phase compartment 14±1%, and the mean G₂ +M-phase compartment 4±0.7%. The percentage of cells in the S-phase had a positive correlation with the LDHlevel, but not with other parameters. Patients at relapse had significantly lower percentages of cells in the S-phase in AMLand significantly higher in ALL. The changes of the S-phase size of the same patients at the first and relapse phases varied. Patients who obtained complete remission had significantly higher percent S phase than patients who did not. This difference was more significant in ALL. Percent cytoreduction had no correlation with the remission rate or S-phase size. Therefore, we analyzed changes of cells in the S-phase of patients with untreated AMLduring chemotherapy. The reduction rate of cells in the S-phase of patients who obtained complete remission was significantly higher than that of patients who did not.