Stroke, silent cerebral infarct (SCI), and cognitive impairment are frequent and highly morbid complications of sickle cell disease (SCD) in children. Current approaches to the prevention and treatment of neurological complications in SCD include screening by transcranial Doppler ultrasound (TCD) to identify children with elevated cerebral blood flow velocity who are at increased risk for strokes; these children are then typically treated with chronic transfusions indefinitely. Hydroxyurea (HU) may have beneficial effects on central nervous system (CNS) complications in SCD and reduces the frequency of painful crisis, acute chest syndrome and transfusion. The safety of HU in infants and children has been suggested in a National Institutes of Health (NIH) sponsored phase III trial; however, the exact indications for the use of HU in children remain unclear, as well as its efficacy in preventing central nervous system (CNS) complications of SCD. Our preliminary data suggest that, if the cumulative frequency of abnormal TCD, SCI and stroke could be reduced by 50%, the majority of pediatric hematologists would prescribe HU to all young children with SCD. The long term goal of this project is to perform a primary prevention trial to demonstrate the neuroprotective effect of HU and broaden the indications for HU in children. The goals of this proposal are to: 1) conduct an internal pilot randomized placebo-controlled trial of HU to reduce the CNS complications of SCD (the term internal pilot is used, as the results from the participants in the pilot will be analyzed as part of a definitive phase III trial to follow); 2) demonstrate the safety of hydroxyurea and study procedures in young children with SCD; and 3) create the leadership, network of clinical centers and other procedures necessary to conduct a definitive phase III trial demonstrating the efficacy of HU for primary prevention of the neurological complications of SCD.