An aneurysm of the pulmonary artery is a rare entity, but has a potentially fatal prognosis. We report a case of a 28 year-old asymptomatic woman, with an idiopathic pulmonary artery aneurysm. She was admitted to our hospital because of an abnormal bulging contour of the left hilum on chest X-ray. A computed tomographic scan and pulmonary angiography showed dilatation of the main and proximal left pulmonary arteries. She has a stable, non-progressive condition without treatment during the 8 month follow-up period.