 The purpose of this study is to assess prospectively the efficacy of hydroxyurea therapy in the setting of cerebrovascular disease, manifest as conditional or abnormal transcranial doppler ultrasonography TCD flow velocities, in children with sickle cell anemia SCA. TCD is used to measure flow velocity in intracranial arteries as a marker of increased stroke risk in children with SCA. The primary objective of this protocol is to determine whether hydroxyurea reduces elevated TCD velocity. 