Obstructive and restrictive pulmonary changes develop in children with sickle cell disease, but reports conflict as to the type of change that predominates. We prospectively performed spirometry, plethysmography, and lung diffusing capacity in 146 children aged 7 to 20 years with hemoglobin SS or Sβ(0)-thalassemia. Nineteen percent of the patients had obstructive physiology as defined according to guidelines of the American Thoracic Society. In addition, 9% had restrictive physiology and 11% had abnormal but not categorized physiology. Increasing age, patient-reported or family-reported history of asthma or wheezing, and higher lactate dehydrogenase concentration were independent predictors of obstruction as reflected in lower forced expiratory volume in the first second/forced vital capacity. In conclusion, abnormal pulmonary function, most often obstructive, is common in children with hemoglobin SS and Sβ(0)-thalassemia. Full pulmonary function testing should be performed in children with hemoglobin SS or Sβ(0)-thalassemia, especially with history of asthma or wheezing and accentuated elevations in hemolytic markers.