The pathogenesis of acute painful crisis in children with sickle cell disease is poorly understood; suggested risk factors include sickle cell type, severity of anemia, fetal hemoglobin concentration, and hypoxemia from upper airway obstruction. In a cohort study of 95 patients the relationship between clinical, laboratory, and sleep study data and frequency of painful crisis was investigated. Both univariate and multiple regression modeling showed that low nocturnal oxygen saturation was highly significantly associated with a higher rate of painful crisis in childhood (P <.0001). Screening and treatment for hypoxemia may reduce the frequency of this and other complications of the disease.