From animal experiments (n = 10), the authors describe a new technique of pulmonary arterial angioscopy. A balloon-fitted catheter is introduced into the trunk of one of the pulmonary arteries. A small caliber angioscope with an end capable of being propped up is then pushed within the lumen of the catheter. Intraluminal visualization of the pulmonary artery and its branches cannot begin until after the balloon is inflated to arrest blood flow and the visualization chamber is cleared by sterile carbon dioxide. The quality of the image obtained is similar to that of bronchial endoscopy, making is possible to examine at length the walls of the proximal and distal pulmonary arteries up to a caliber of 2.5 mm, and to detect blood clots. Pulmonary angioscopy is tolerated to the same extent as right heart catheterization. Potential applications of this technique are diagnosis of chronic cor pulmonale and treatment of pulmonary embolism.