The primary endpoint for the internal pilot and definitive phase III trials will be the development of abnormal TCD, SCI, TIA or stroke. To begin the internal pilot trial, the investigators obtained Clinical and Translational Science Award (CTSA) support at Johns Hopkins and Washington University; these sites will screen 40 participants 9-48 months of age and randomly assign and follow 20 participants for three years. Four additional centers (Children's Hospital of Philadelphia, Vanderbilt University,Children's Hospital Medical Center, Cincinnati and the University of Alabama, Birmingham) began enrollment (up to 20 patients screened and 10 participants randomly assigned per site), to provide a total of 80 participants screened, 40 randomly assigned, and a minimum of 70 participant years of follow-up. Additional sites have been added. Participants must have TCD measurements that are well below transfusion thresholds and magnetic resonance imaging (MRI) of the brain without evidence of SCI. Participants in the internal pilot will continue into a phase III trial, to complete 3 years on HU or placebo. The information from the internal pilot trial will be used to improve the design of the definitive phase III trial. The results of these studies could lead to true primary prevention of CNS complications of SCD, including abnormal TCD, SCI, neurocognitive impairment and stroke. In doing so, this study could also reduce the burden of chronic transfusions and change clinical practice by broadening the indications for HU.