The influence of excessive weight loss, with decreased muscle mass, on pulmonary function was evaluated in 16 obese patients without clinical evidence of respiratory disease. Weight loss was induced with vertical banded gastroplasty. Six nonobese age-matched patients undergoing elective cholecystectomy acted as controls. Preoperative values for static and dynamic lung function variables and for pulmonary gas distribution and respiratory muscle strength were compared with postoperative values after approximately 10% and 18% weight reduction. Before gastroplasty the dominant abnormality of respiratory function was slight restrictiveness, with reduced lung volumes. Body weight and lean body mass decreased significantly during the study period, and respiratory muscle strength was significantly reduced after 10% weight loss. Lung function tests nevertheless showed significant increase of vital capacity, functional residual capacity, total lung capacity and maximal voluntary ventilation. No correlation was found between the reduction of respiratory muscle strength and that of lean body mass. The controls did not diminish the body weight after surgery and the lung volumes and respiratory muscle strength was not changed.