The vast majority of births with sickle cell disease (SCD) occur in Africa and 90% are thought to die before the age of five. Hydroxyurea (HU) is the only drug approved by the FDA for the treatment of sickle cell anemia. Although HU is used to treat small numbers of patients in Africa, cost, fear of toxicity, and lack of awareness and availability limit its use. The leukopenia that may be seen with HU raises the possibility of increased susceptibility to infection. Risk stratification - i.e., identification of patients most likely to benefit- could focus therapy and provide confidence that the risk:benefit ratio is favorable. Several clinical measures of future risk are well defined and findings on modifier genes in the US, primarily related to fetal hemoglobin (HbF), have further improved risk prediction. Whether the genetic variants predict severity in Africa is not known. The investigators have established a SCD cohort in Ibadan, Nigeria. In the first phase of this research the investigators will implement clinical risk examinations and assess the relationship between clinical characteristics (including levels of HbF) and known genetic markers. As a proxy for a birth cohort, the investigators will compare the frequency of the genetic markers in adult patients (i.e., "survivors") to children. In the second phase the investigators will randomize 40 high risk adult patients to fixed low dose HU or no HU treatment in a crossover design and monitor hematologic and physiologic parameters to document hematologic effects and safety. This work will lay the basis for a large-scale trial to document safety and efficacy.