 This is a phase II, prospective, randomized, double-blind, placebo-controlled, parallel-group,study to evaluate the safety and efficacy of oral L-glutamine therapy for patients with sickle cell anemia or sickle IS-thalassemia who are at least 18 years old with focus on the aspect of exercise endurance. In this study, the patients will orally take L-glutamine or placebo twice daily, and clinical and hematological parameters will be monitored. The adverse events,especially those attributable to L-glutamine therapy, will also be monitored. The data obtained will be essential in understanding the role of L-glutamine in therapy for sickle cell anemia and sickle B-thalassemia. 