The response of pulmonary arterial pressure to minor degrees of pulmonary embolism was examined in 18 patients with embolic occlusion of less than 25% of the pulmonary vascular bed. Patients with pulmonary embolism were compared to normal controls matched for age and sex and to patients with a variety of acute pulmonary disorders without pulmonary embolism. Patients with pulmonary embolism and patients with other acute pulmonary diseases had significantly higher pulmonary arterial pressures and significantly lower values for arterial oxygen tension (PaO2) than did normal subjects. The degree of pulmonary hypertension correlated with the PaO2. Pulmonary hypertension occurring after minor degrees of pulmonary embolism may be a response to mild arterial hypoxemia.