Objective: To determine the prevalence and reversibility of lower airway obstruction (LAO) in children and adolescents with hemoglobin SS sickle cell disease (HbSS SCD). Study Design: Retrospective evaluation of lung function in a cross-section of 35 African American and 28 Hispanic children and adolescents with HbSS SCD. Lung function was evaluated with maximal respiratory flow-volume curves and body plethysmography. Each patient was assigned to 1 of 3 patterns of lung function (normal, obstructive, or restrictive). Airway hyperresponsiveness was assessed by means of a trial with bronchodilator. Results: Normal pattern was detected in 57% of the patients, LAO in 35%, and restrictive lung disease in 8%. Positive response to bronchodilator was documented in 30% of those with normal pattern of lung function, 78% in those with LAO, and 67% of those with restrictive lung disease. The pattern of lung function was not associated with race or with history of vaso-occlusive crises, acute chest syndrome, reactive airways disease/asthma, or long-term transfusion therapy. Conclusion: Obstructive lung disease possibly precedes the development of restrictive lung disease, and airway reactivity may be part of the pathogenic mechanism. (J Pediatr 2001;138:188-92)