We evaluated the role of fiberoptic angioscopy in eight patients with suspected chronic pulmonary embolic obstruction of the pulmonary arteries. Angioscopy, preceded by ventilation-perfusion lung scans, right-heart catheterization, and pulmonary angiography, resulted in diagnostic changes in four patients: from pulmonary artery agenesis to chronic emboli; from chronic emboli to normal pulmonary arterial intima (primary pulmonary hypertension); from chronic pulmonary emboli to extrinsic compression of a major pulmonary artery (fibrosing mediastinitis); and from suspected agenesis or chronic emboli to a tumor (fibrosarcoma) of the pulmonary artery. Angioscopy also more accurately determined the extent and surgical accessibility of chronic embolic obstruction in the five patients with that disorder; as a result, we decided that the obstruction in one patient was inoperable. No significant complications occurred with angioscopy and we conclude that its direct visualization capability can contribute significantly to the diagnostic evaluation of suspected chronic pulmonary arterial obstruction.