Sequence I (Standard): When the fetal hemoglobin levels have been stable for 2 months, patients have a repeat echocardiogram and 6-minute walk test. Erythropoietin is then added to the hydroxyurea regimen. It is given 3 days a week, as an injection under the skin, along with iron supplements. Patients have blood tests and blood pressure measurements every week or every other week. Patients with pulmonary hypertension have another echocardiogram and 6-minute walk test once the hemoglobin level is stable.