The effect of obesity on pulmonary function was studied in 13 children, aged 8-15 years, with 147-300% ideal body weight (IBW). Measurements included lung volumes, airflow rates pre- and post-bronchodilator nebulization, diffusing capacity (DLCO), maximal voluntary ventilation (MVV), minute ventilation VE), and resting energy expenditure (REE). When compared with predicted normal values for sex, height, and body surface area (BSA), decreases (mean % predicted, +/- SE) were observed in expiratory reserve volume (ERV, 36 +/- 5); forced expiratory volume in 1 second (FEV1, 73 +/- 5); forced expiratory flow between 25% and 75% of vital capacity (FEF25-75%, 70 +/- 6); DLCO, absolute (52 +/- 3) and corrected (DLCO/VA, 71 +/- 5); and MVV (62 +/- 5). Residual volume (RV), RV/total lung capacity (TLC), VE, and REE were elevated. Other lung volumes were normal. Thus, obese children have altered pulmonary function, which is characterized by reductions in DLCO and ventilatory muscle endurance and airway narrowing. These alterations may reflect extrinsic mechanical compression on the lung and thorax, and/or intrinsic changes within the lung. The reduced DLCO may result from decreases in alveolar surface area relative to lung volume.