The applicability of angioscopy for observation of the pulmonary arterial lumen was examined in 7 anesthetized dogs and in 10 patients with various heart diseases during cardiac catheterization. A novel guiding balloon catheter (#11F shaft diameter and 1.5 cm in balloon diameter) was introduced through the right femoral vein into the pulmonary artery by Seldinger's method. Then, a fiberscope (1.3 or 1.8 mm in diameter) was introduced through the guiding catheter into the desired portion of the artery. The balloon was inflated, saline was infused, and the pulmonary arterial lumen was photographed on 16-mm color cinefilms. In all dogs and patients examined, an increase in diameter of the pulmonary artery during systole and the backflow of blood from the pulmonary vein were clearly demonstrated. Also, pulmonary thrombi or emboli experimentally induced in dogs could be observed. No complications were observed in the patients. The results indicate the experimental and clinical applicability of angioscopy using a guiding balloon catheter for observation of the pulmonary arterial lumen.