Randomized, Phase III, multicenter. Approximately 3,000 children from 12 clinics were screened with transcranial Doppler (TCD). A total of 130 were randomized to receive either standard supportive care or periodic blood transfusions if they were found to be at high risk of stroke on the basis of elevated cerebral blood flow as measured by TCD screening tests. Primary endpoints included clinically evident symptoms of cerebral infarction with consistent findings on magnetic resonance imaging (MRI), and/or symptomatic intracranial hemorrhage. Secondary endpoints included asymptomatic brain lesions detected by MRI in brain areas not involved in primary endpoints. Hematologic characteristics of the high risk group were analyzed and serum and DNA samples frozen for future analysis. Recruitment ended in October 1997 with the accrual of 130 subjects. The clinical phase ended in 1999.