This was a 2:1 randomized, double-blind, placebo-controlled, parallel-group, multicenter study in patients with sickle cell anemia and sickle ß0-thalassemia who were at least 5 years old. Informed consent was obtained up to four weeks prior to Week 0 (Baseline). Screening procedures were performed anytime between the date of consent and Week 0, as long as all eligibility criteria had been confirmed prior to Week 0. At Week 0, patients were randomized (to L-glutamine or placebo) and underwent 48 weeks of treatment (orally BID), with dose calculated according to patient weight. Patient clinic visits occurred every 4 weeks, and phone calls took place between visits to monitor compliance. After 48 weeks of treatment, the dose was tapered to 0 within 3 weeks. A final evaluation visit occurred 2 weeks after last dose for a total of 53 weeks on study.