Diagnosis of sickle cell anemia with clinically severe disease manifestations defined by: Recurrent painful events (at least 2 painful events in past year) which cannot be explained by other causes Pain lasts at least 4 hours Requires treatment with parenteral narcotics, equianalgesic dose of oral narcotics, or parenteral nonsteroidal antiinflammatory drugs Acute chest syndrome (ACS) with at least 2 episodes within past 2 years that required hospitalization, oxygen, and RBC transfusion Any combination of painful events and ACS episodes that total 2 events within the past year Abnormal cerebral MRI, abnormal angiography (MR or conventional), and abnormal neuropsychologic testing performance