DNA‐synthesis stage and total number of circulating burst‐forming‐units‐erythroid (BFU‐E) have been inversely correlated with hemoglobin F levels in the peripheral blood, as well as in the cells from the BFU‐E‐derived colonies obtained from homozygous sickle cell anemia (SS) patients during steady state. Similar studies in SS patients treated with cytotoxic agents have not been reported. However, regeneration of the erythroid marrow that follows the cytoreduction phase of chemotherapy has been suggested as one of the mechanisms of stimulation of fetal hemoglobin synthesis. Therefore, a longitudinal study of hemopoiesis in hydroxyurea‐treated SS patients was conducted. Thirty‐two sets of hemopoietic studies, including total circulating BFU‐E and S‐phase BFU‐E, were obtained from three patients treated with hydroxyurea. A dose‐dependent decrease in total BFU‐E colonies occurred in peripheral blood of all three patients (r = −0.58, −0.85, and −0.97, respectively, with each P < 0.05). There was a strong positive correlation between hydroxyurea dose and fetal hemoglobin levels in two of the three patients who responded clinically (r = 0.89618 and 0.88632, respectively, with each P < 0.01). When data from all patients were combined (n = 32), there was a strong, inverse, linear relationship between total number of BFU‐E and percentage S‐phase BFU‐E with fetal hemoglobin levels (r = −0.6649 and −0.7404, respectively, with each P < 0.0001). A stronger, curvilinear, multiple relationship was detected between total BFU‐E and percentage S‐phase BFU‐E with fetal hemoglobin levels (R = 0.8351 and 0.8602 with each P < 0.0001). © 1994 Wiley‐Liss, Inc.