Stroke Prevention in Nigeria 2 Trial

Inclusion Criteria: The inclusion criteria for the SPRING-2 Trial will consist of: * Diagnosis of HbSS or HbSB0 confirmed by high-performance liquid chromatography (HPLC); * Informed consent from the parent/legal guardian and assent from the patient at least 7 years of age; * Two TCD flow velocity readings of \>or equal to180 cm/sec and \< 220 cm/sec or one TCD velocity reading \> or equal to 220 cm/sec; typically the repeat TCD is performed on the same day so treatment can start immediately; * Age between 5 and 12 years (assessment can take place up until the 13th birthday), which includes the peak age of onset of strokes in SCA, \~ 6 yo; and * Ability to swallow the hydroxyurea capsule. Exclusion Criteria: The exclusion criteria will be the following: * Prior stroke or TIA by history, or concern for moderate or severe neurological deficit based on a positive validated "10 questions" screening; * Other significant organ system dysfunction or other contraindication to hydroxyurea; * Children who are already on therapy with either blood transfusion or hydroxyurea therapy; * Significant cytopenias (absolute neutrophil count (ANC) \<1500, platelets \<150,000/ul, reticulocytes \<80,000/ul, unless Hb is \> 9 g/dl\], renal insufficiency (creatinine \> 0.8 mg/dl); and * History of seizures or diagnosis of epilepsy, and 6) metal in the body that would make MRI unsafe. The rationale for excluding children under 5 years old: Despite being a vulnerable age group for strokes, children younger than 5 years were excluded because a significant proportion of this population is unable to swallow a capsule, the only stable form of hydroxyurea available in Nigeria.