Preliminary studies by the investigators and others have shown convincingly that Hb F levels can be increased by administering hydroxyurea to patients over a several month period without producing dangerous levels of myelosuppression. The patients have also had a rise in their red cell life span and hematocrit. Further, it has been reported that these patients had a dramatic decrease in crisis frequency. This agent is readily available to all physicians, and there is evidence that it is being used without adequate justification and, possibly, without adequate monitoring. The timing of this study is therefore critical to ensure that an adequate answer to the question of efficacy is obtained and the risk of inappropriate use minimized.