The results of 64 membranes diffusing capacity (Dm) and pulmonary capillary blood volume (Vc) estimations were analysed to assess the clinical significance of these measurements. These estimations were performed on 18 healthy subjects, 19 patients with mitral valvular involvement of rheumatic aetiology, 15 patients with chronic bronchitis and emphysema, 8 patients with chronic bronchitis alone, and 4 patients with pulmonary embolism. It was observed that Dm correlated very well with the pulmonary diffusing capacity (DLCO) measured during the inhalation of room air in all the groups of subjects. In patients with rheumatic heart disease, the DLCO was affected little by even large changes in Vc, whereas it ran closely parallel to the Dm in these subjects. In the past Dm has been considered to be an unreliable estimation, varying greatly as a result of small errors in the measurement of DLCO. This appears to be relatively true only in normal subjects having low Vc/Dm ratio. In a majority of diverse clinical conditions where the Vc/Dm ratio is increased, the Dm becomes a more reliable estimation. In these patients the DLCO itself is a good index of the membrane diffusing capacity.