The pulmonary diffusing capacity for carbon monoxide was measured by single breath (DcoSB) and steady state (DcoSS) techniques in 48 patients with various pulmonary circulatory disorders and the results compared with the type of hemodynamic abnormality. Patients with pure inflow obstruction (primary pulmonary hypertension, recurrent pulmonary emboli) had the lowest Dco (mean DcoSB = 14.3 ml/min/mm Hg [59%], mean DcoSS = 9.0 ml/min/mm Hg [58%]). Patients with inflow and outflow obstruction, (valvular heart disease with increased pulmonary vascular resistance [PVR]), had higher values (mean DcoSB = 16.2 [68%], mean DcoSS = 10.2 [65%]). In patients with pure outflow obstruction (valvular heart disease and normal PVR) Dco was normal (means 23.2 [96%] and 14.4 [87%] respectively). Patients with increased pulmonary blood flow (PBF), (left to right shunts), had the highest Dco (means 31.3 [118%] and 16.4 [106%]). There was a reciprocal relationship between Dco and PVR, a low Dco being characteristic of pulmonary inflow obstruction. Patients with increased PBF had lower Dco values than exercising normal subjects with comparable PBF. In patients with pulmonary circulatory abnormalities and no obstructive or restrictive lung disease, Dco is low when inflow obstruction predominates. Dco inceases with the addition of outflow obstruction and is above normal when there is an increased PBF and low PVR.