A fifty-five-year-old woman had worried about dyspnea for three years. A chest x-ray appeared normal except for hypovascularity of the upper lung field. On angiograms, a wedge-shaped obstruction was observed at the left proximal pulmonary artery, and well-developed collateral circulation from the right thoracic artery to the internal costal arteries was observed. On the exercise test, exertional dyspnea developed with tachycardia and a decreased saturation of arterial oxygen.