In this multicenter, randomized clinical trial, 100 children will be randomized to continue to receive periodic transfusion therapy and 50 to discontinue receiving periodic transfusion therapy. The primary endpoint will be the reversion of the transcranial Doppler velocity levels to greater than 200 cm/second, indicating return of increased risk for overt stroke. Recruitment will be in two phases. Phase I will include those patients who began transfusions before April 1, 1999. These will come mainly from the STOP 1 cohort. Those who begin transfusions after April 1, 1999 but before April 1, 2001 will be eligible for Phase II of recruitment. All patients will receive quarterly TCD examinations. Patients who revert to high risk will be offered return to transfusion. The overall design includes a three month start-up, the two phases of recruitment (established STOP patients and new patients) for a total of 36 months, 18 months of observations after recruitment ends and then 3 months of wrap-up.