The clinical and functional features of idiopathic fibrosing alveolitis (IFA) were analyzed in 31 patients. In 24 patients, the diagnosis was confirmed by histological study of the pulmonary tissue obtained by means of open lung biopsy or at autopsy. Progressive dyspnea was the earliest clinical symptom of IFA. High temperature, weight loss, cyanosis and other symptoms were characteristic of an acute disease course. Crepitation was the most characteristic symptom as far as the objective data are concerned. Detailed examination of respiratory function with the use of spirography, general plethysmography, forced oscillation technique, study of the diffuse capacity of the lungs and blood gases, and other methods showed that the most characteristic functional features of pulmonary respiration in IFA involved a decrease in the static pulmonary volumes, an increase in elastic pulmonary resistance, normal patency of the large and medium bronchi, and a diminution of the diffuse capacity of the lungs during respiratory arrest, as well as that of the diffuse capacity of the alveolocapillary membrane. The data obtained indicate that today one can discuss only the most characteristic rather than the pathognomonic clinical and functional symptoms of IFA.