Stroke Prevention With Hydroxyurea Enabled Through Research and Education (SPHERE)

Inclusion Criteria: * Willingness to sign informed consent * Willingness to follow all study procedures * Available for study visits for the duration of the study and no plans to move away from study center. * Confirmed diagnosis of Sickle Cell Anemia (SCA) by haemoglobin electrophoresis. * Able to take oral medication and follow hydroxyurea treatment schedule. Exclusion Criteria: There are no permanent exclusion criteria for participants to enroll in the screening TCD portion of SPHERE. Temporary, time-limited exclusion criteria for the screening TCD portion include the following: * Febrile illness within the past two weeks. (Temporary Exclusion) * Hospitalized within the past two weeks. (Temporary Exclusion) * Transfusion within the past two weeks. (Temporary Exclusion) Patients who enroll in the screening portion, have a conditional or abnormal TCD, and are eligible to start hydroxyurea will be excluded from receiving study treatment if they meet any of the following criteria: * Abnormal pre-enrolment laboratory values (Temporary Exclusion) * Known medical condition making participation ill-advised. * Known allergic reactions to components of hydroxyurea. * Previous history of stroke. * Currently pregnant or lactating.