Pulmonary function patterns were found to be different in patients with "pure" mitral stenosis (= 29), with concomitant mitral incompetence (n = 18) and with coronary heart disease (n = 12). Lung function in "pure" mitral stenosis has a trend to increased airway resistance with low vital capacity, without any statistical correlation between lung function data and hemodynamics. Coronary heart disease, in contrast, was found to show lung function abnormalities indicating increased pulmonary capillary volume and passive pulmonary hypertension, both being compatible with latent congestive failure. The group with combined mitral valve disease represented an intermediate type of cardiac lung. It can be concluded that a reflex mechanism is important in mitral stenosis between interstitial lung tissue and bronchomotor tone, but not necessarily between left atrium and pulmonary arteriolar tone. This reflex apparently is weaker in combined mitral valve disease and practically absent in coronary heart disease, where the expected lung function pattern compatible with early congestion should be encountered. The different pattern of mitral stenosis seems however, not to be related to the valve stenosis, but to the longstanding increase in pressures. The same mechanisms have been demonstrated in patients with acute left heart failure.