Previous approaches to the therapy of this group of disorders have included: attempts to modify the hemoglobin molecule so as to prevent polymerization; the use of vasoreactive drugs; and increasing red cell volume. All of these attempts have been abandoned either because of their inefficacy, toxicity, or impracticality. Chronic exchange transfusion programs have been of limited usefulness because of high rates of isoimmunization, iron overload, and risk of transmission of hepatitis and retroviral disease.