Hydroxyurea (HU) is the first drug that, under well-organized clinical trials, has shown the potential for altering significantly the clinical severity of sickle cell disease (SCD). The placebo-controlled trial of HU in adult SS hemoglobinopathy patients (the Multicenter Study of Hydroxyurea in Sickle Cell Anemia) reported in May 1995 that HU therapy reduced significantly the frequencies of severe pain episodes, acute chest syndrome, and transfusion. Despite these impressive results, no guidelines have been developed to direct clinicians on the use of HU in adult SCD patients. Small-scale phase II studies in children have reported increases in fetal hemoglobin (HbF) and F-cell levels in response to HU therapy. A larger phase II study, the Pediatric Hydroxyurea Study Group (HUG-KIDS), is under way and is expected to be completed by March 1998. The need for a large-scale placebo-controlled trial in children will be doubtful if no unusual short-term toxicity is demonstrated by HUG-KIDS. Guidelines regarding patient selection, dosing schedules, treatment goals, and short- and long-term monitoring parameters need to be established. The case is made of the organization of a clinical network to register and follow SCD patients treated with HU for long-term toxicity.