Decrease in the transfer of gases through the alveolar capillary membrane can be produced by alterations of the alveolar capillary membrane, such as edema or thickening or by a marked decrease in the size of the alveolar capillary membrane available for diffusion of gases. Restrictive and obstructive disease were present in all the patients with pulmonary fibrosis and lung carcinoma, 4 patients with treated sarcoidosis had normal spirometry and diffusion capacity. Correlation coefficients were high between mean vital capacity before and after bronchodilators in the 3 groups. The correlation coefficients were also between mean forced vital capacity in first, second before and after bronchodilators. The correlation coefficient between vital capacity either before or after bronchodilators and the diffusion capacity steady state was the highest in the patients with pulmonary fibrosis, r = 0.7275 and r = 0.7238, respectively; indicating that in the other patients, factors other than a reduction in the alveolar capillary surface available for diffusion of gases caused the reduction in the diffuse capacity.