The clinical course of most patients with pulmonary embolism is one of gradual resolution with re-establishment of flow in the pulmonary arteries. In a small but definite group of patients, the emboli do not resolve and a state of chronic pulmonary embolism ensues. The primary thrombotic process in the systemic venous system may persist, and in some instances may be unrecognized. Such patients experience recurrent showers of emboli which may ultimately occlude a large part of the pulmonary arterial circulation with development of severe respiratory insufficiency. Six patients with this syndrome are described, and in each there was a history of dyspnea, cyanoiss, and exercise intolerance associated with a low arterial PO2, right ventricular hypertrophy, and pulmonary hypertension. Pulmonary scans and arteriograms demonstrated that more than half of the major pulmonary arteries were occluded and, in addition, smaller vessels were also obstructed. Pulmonary embolectomy was performed in each patient. Five of the 6 obtained a highly gratifying response, including relief of the dyspnea and cyanosis, an increase in arterial PO2, and a decrease in pulmonary arterial pressure. In each of the five in whom improvement occurred, the back-bleeding from the pulmonary artery at the time of embolectomy was quite good. In the sixth patient, the back-bleeding was very poor, and despite embolectomy, the vessel thrombosed postoperatively with no improvement in the patient's clinical course. Follow-up studies in these patients range up to 8 years with demonstration of continued patency of the pulmonary arteries as well as continued improvement in clinical symptoms and in the arterial PO2.