--Disease Characteristics-- Sickle cell syndromes characterized by vaso-occlusive crises sufficiently severe to require prophylactic therapy Three or more crises per year sufficiently severe to require hospitalization Stable dose of hydroxyurea for at least 4 months required --Prior/Concurrent Therapy-- No treatment with any other antisickling agents within the past 4 months Biologic therapy: No transfusion within 90 days No concurrent chronic transfusions allowed (defined as more than one transfusion per month for 2 or more months) Chemotherapy: See Disease Characteristics Endocrine therapy: Not specified Radiotherapy: Not specified Surgery: Not specified Other: No chronic medications that alter neurologic, renal, or hepatic functions