The level to which the diaphragm descends on inspiration is dictated by the opposing forces of muscular inspiratory effort and elastic recoil of the lungs, best measured and expressed as its reciprocal; compliance. The level to which the diaphragm ascends on expiration is not dictated by muscular effort but by the elasticity of the chest wall and again by the pulmonary compliance. We theorized therefore that, if there is no change in inspiratory effort or chest wall tonus, the diaphragmatic excursion must be related to the functional or dynamic compliance of the lung. To test this theory inspiratory/expiratory roentgenograms have been taken at total lung capacity and functional residual capacity, respectively, and pulmonary function studies including mechanics (compliance studies) performed on 250 patients. Of these, 130 had chronic obstructive pulmonary disease, 50 had sarcoidosis, 20 renal failure, 40 rheumatic valvular disease, and 10 were controls. A further 900 patients had inspiratory/expiratory roe...