Phase I has concluded. Phase II, also concluded, was a randomized, double-blind, placebo-controlled trial. Patients in the Phase II trial were recruited from 21 clinics and randomized to receive hydroxyurea or placebo. The hydroxyurea was gradually increased from an initial dose of 15 mg/kg to the maximal level tolerated by each patient in order to maximize red blood cell hemoglobin F(Hb F) content without undue marrow suppression. Changes in Hb F production were monitored in each of the two groups by a variety of laboratory tests. The primary endpoint was a comparison of crisis rates in the treated and control groups. Painful crises were defined as pain lasting longer than four hours, requiring parenteral narcotics for relief, including chest syndrome but excluding ankle ulcer pain. Secondary endpoints included changes in pain severity and duration, psychosocial status, complications of the disease, and reasons for non-compliance with either regimen. Patients were followed for two to three years depending on when they entered the study. Because of the mutagenic nature of hydroxyurea, the use of contraception was a requirement of admission to the study.