Erythrocyte dehydration is an important feature of sickle cell disease, leading to increased sickle hemoglobin polymerization and decreased red blood cell survival. Substantial in vivo dehydration appears to occur in reticulocytes or in an even younger subset of reticulocytes that are positive for transferrin receptor. Previous studies have suggested both sickling-dependent and sickling-independent components of dehydration for these cells. Two types of investigations are reported here. The first series of experiments explored the possibility that fetal hemoglobin (HbF) content influences the in vivo dehydration of very young, transferrin receptor-positive (T+) cells. These studies confirmed that in most patients the T+ cells in the densest fraction lacked HbF (T+ F-). However, T+ F- and T+ F+ cells appeared to have the same tendency to become moderately dense. The second type of investigation examined moderately dense T+ cells with normalized K+ content and determined the effect of HbF content on KCl cotransport-mediated dehydration in oxygenated incubations. Under these conditions, both T+ F- and T+ F+ cells had an equal tendency to become more dense by this pathway. Taken together, these studies indicate that at least some young sickle cells become moderately dense due to higher KCl cotransport activity independent of HbF content (and by inference, independent of sickling). However, to become very dense, it appears that further dehydration through a sickling-mediated pathway is required. We suggest that the dehydration of young sickle cells occurs in two steps, with the first dominated by KCl cotransport and the second having an important sickling-dependent component.