Partial exchange transfusion is used frequently in sickle cell anemia when surgery is indicated, when life-threatening complications (for example bilobar pneumonia) occur, or when a patient is totally incapacitated from recurring crises. Because either fresh or stored blood has higher oxygen affinity then sickle cell blood, infusion of large amounts of such blood could cause dramatic alteration of the recipient's blood oxygen affinity. The present study has indicated that partial exchange transfusion with maintenance of hemoglobin concentration and blood volume results in improved exercise tolerance. We interpret this to mean that the flow properties of sickle cell blood through peripheral capillary beds may be more important in determining oxygen delivery than oxygen affinity of the red cells themselves.