Homozygous beta thalassemia Other beta chain mutation as demonstrated by hemoglobin electrophoresis Sickle cell anemia (age 16 to 50 years) Evidence of one or more the following prior complications: Stage I-II sickle cell lung disease Sickle cell nephropathy (moderate to severe proteinuria or glomerular filtration rate of 30-50% predicted normal for age) Acute chest syndrome requiring blood transfusions More than 3 debilitating pain episodes per year for 3 years lasting more than 4 hours each Any combination of episodes of acute chest syndrome and painful episodes adding up to 3 episodes per year for 3 consecutive years Requirement for chronic transfusions with alloimmunization (more than 2 antibodies) Sickle cell anemia (age under 16 years) Evidence of one or more the following prior complications: Prior stroke or hemorrhage Any neurologic event lasting more than 24 hours Abnormal cerebral MRI and cerebral arteriogram MRI angiographic impaired neuropsychologic testing Stage I-II sickle cell lung disease Sickle cell nephropathy (moderate to severe proteinuria or glomerular filtration rate of 30-50% predicted normal for age) Significant visual impairment in at least one eye with bilateral proliferative retinopathy Acute chest syndrome with history of recurrent hospitalizations or exchange transfusions Osteonecrosis of multiple joints with destructive changes More than 3 debilitating pain episodes per year or priapism Requirement for chronic transfusions with alloimmunization