To clarify the mechanism of the disturbance of alveolar gas exchange in idiopathic pulmonary fibrosis, various pulmonary function tests were performed on 7 patients with the disease, of which 5 were defined histo-pathologically and 2 clinically. On 4 of all patients, detailed comparative studies of histopathological findings to pulmonary functions were carried out additionally. As the results, marked decreases of total lung capacity, pulmonary diffusing capacity (DLco) and Pao2 were observed. On exercise, the improvement of DLco or physiologic dead space (VD/VT) was hardly seen, while AaDO2 increased markedly. Ventilation (VA), perfusion (Q), and VA/Q ratio examined by 133Xe scintigram distributed more evenly over the entire lung fields than controls. These may suggest that the disturbance of alveolar gas exchange is mainly due to the elongation of the distance for diffusion in the membrane.