The increase in fetal-hemoglobin synthesis in patients with beta-thalassemia or sickle-cell anemia induced by 5-azacytidine has been attributed to hypomethylation of DNA in the region of the gamma-globin genes. To determine whether hydroxyurea, a cytotoxic/cytostatic drug that does not influence DNA methylation, might stimulate fetal-hemoglobin synthesis, we phlebotomized two juvenile cynomolgus monkeys to induce anemia and reticulocytosis and then treated them with hydroxyurea. Immediately after phlebotomy was initiated, there was a rise in the level of F cells, which stabilized at an average value of 13 per cent in one animal and 20 per cent in the other during a two-month control period. Fetal hemoglobin gradually rose from undetectable values before bleeding to 3 per cent in one animal and 5 per cent in the other. Sixty-two days after initiation of phlebotomy, hydroxyurea (50 mg per kilogram of body weight per day for five days) induced only a small and transient increase in F cells and fetal hemoglobin. Two weeks later, however, a similar course (100 mg per kilogram per day) resulted in a prompt and dramatic increase in both indexes. These results strongly suggest that S-phase-specific cytotoxic/cytostatic drugs increase fetal hemoglobin by a mechanism that does not involve inhibition of DNA methylation.