Activated neutrophils increase erythrocyte phosphatidylserine (PS) exposure. PS-exposed sickle red blood cells (SSRBCs) are more adhesive to vascular endothelium than non-PS-exposed cells. An increase in SSRBC fetal hemoglobin (HbF) concentration has been associated with improved rheology and decreased numbers of vasoocclusive episodes. This study examined the effects of HbF, PS-exposed SSRBCs, and chronic hydroxyurea (HU) treatment on activated neutrophil-mediated SSRBC retention/adherence in isolated-perfused rat lungs. Lungs were perfused with erythrocyte suspensions from 1) individuals homozygous for hemoglobin S with 0-7% HbF (SS), 2) with > or =8% HbF (SS + F), and 3) individuals homozygous for hemoglobin S treated with HU therapy for > or =1 yr (SS + HU). Retention of SSRBCs from the SS + HU group was significantly less than that seen in both the SS and SS + F groups. No difference was observed between the SS and SS + F groups. The percentage of HbF and F-cells did not differ between the SS + F and SS + HU groups. At baseline, the proportion of PS-exposed SSRBCs was not different between the SS and SS + HU groups. However, SSRBC treatment with activated neutrophil supernatant caused a twofold increase in PS-exposed SSRBCs in the SS control and no change in the SS + HU group. We conclude that 1) HU attenuates SSRBC retention/adherence in the pulmonary circulation seen in response neutrophil activation, 2) HU stabilizes SSRBC membrane PS, and 3) HU attenuation SSRBC retention/adherence in the pulmonary circulation occurs through a mechanism(s) independent of HbF.