The determinants of sickle red blood cell (RBC) life span have not been well-defined but may include both intrinsic factors (eg, the tendency to sickle) and extrinsic factors (eg, the capacity of the reticuloendothelial system to remove defective RBCs). Fetal hemoglobin (HbF) is heterogeneously distributed among sickle RBCs; F cells contain 20% to 25% HbF, whereas the remainder have no detectable HbF (non-F cells). Autologous sickle RBCs were labeled with biotin and reinfused to determine overall survival, non–F- and F-cell survival, and time-dependent changes in HbF content (%HbF) for the surviving F cells. A total of 10 patients were enrolled, including 2 who were studied before and after the percentage of F cells was increased by treatment with hydroxyurea. As expected, F cells survived longer in all subjects. Non–F-cell survival correlated inversely with the percentage of F cells, with the time for 30% cell survival ranging from 6 days in patients with more than 88% F cells to 16 days in patients with less than 16% F cells. As the biotin-labeled RBCs aged in the circulation, the HbF content of the surviving F-cell population increased by 0.28%/d ± 0.21%/d, indicating that within the F-cell population those with higher HbF content survived longer.