Thromboendarterectomy is the treatment of choice for chronic large vessel thromboembolic pulmonary hypertension. To identify the mechanisms responsible for the improvement in gas exchange following thromboendarterectomy, we studied nine patients with chronic thromboembolic pulmonary hypertension before and eight to 18 months after surgery using the multiple inert gas elimination technique. Preoperatively, all subjects had pulmonary hypertension and were hypoxemic or had an elevated P(A-a)O2. The VA/Q distribution was widened with an elevated VD/VT and a low cardiac index. After thromboendarterectomy, significant improvement had occurred. The VA/Q distribution had narrowed to near normal, and the cardiac index increased. It was concluded that thromboendarterectomy improved gas exchange both by improving VA/Q relationships and by increasing cardiac output.