The status of small pulmonary arteries may influence diagnosis, surgical selection and postoperative outcome of patients with chronic major vessel thromboembolic pulmonary hypertension (CTEPH). Therefore, in patients with the established diagnosis of CTEPH, lung tissue was obtained by biopsy (15 patients) or at autopsy (16 patients) to assess the histopathologic composition of small pulmonary arteries. Pathologic examination disclosed the full range of pulmonary hypertensive lesions in the small arteries, including plexogenic lesions. The type and extent of hypertensive lesions did not relate to preoperative hemodynamic values, to patient age, or to symptom duration. The findings indicate that primary pulmonary hypertension cannot be differentiated from potentially correctable CTEPH on the basis of histopathologic findings in small pulmonary arteries. Furthermore, none of the histologic findings preclude a positive hemodynamic and clinical result from pulmonary thromboendarterectomy. However, development of these hypertensive changes may explain the deterioration which these patients experience preoperatively over time.