The Phase I-Phase II study, HUG-KIDS, examined the safety of hydroxyurea. Children with sickle cell anemia, age 5 to 15 years, were eligible for this multicenter Phase I/II trial. Hydroxyurea was started at 15 mg/kg/d and escalated to 30 mg/kg/d unless the patient experienced laboratory toxicity. Patients were monitored by 2-week visits to assess compliance, toxicity, clinical adverse events, growth parameters, and laboratory efficacy associated with hydroxyurea treatment. Eighty-four children were enrolled between December 1994 and March 1996. Sixty-eight children reached maximum tolerated dose (MTD) and 52 were treated at MTD for 1 year. The study was conducted at four Comprehensive Sickle Cell Centers by the following investigators: Thomas R. Kinney at Duke University Medical Center, Durham, North Carolina; Kwaku Ohene-Frempong at Children's Hospital of Philadelphia; Orah S. Platt at Children's Hospital in Boston; and Elliot Vichinsky at Children's Hospital in Oakland, California. The complete study lasted three years.