The authors present a case of a large saccular aneurysm in a coronary artery fistula originating from the proximal left anterior descending coronary artery, draining into the main trunk of the pulmonary artery. The diagnosis was made by 3-dimensional computed tomography and coronary arteriography. Congenital coronary artery fistula is not uncommon. With a saccular aneurysm, however, it is very rare. A ruptured aneurysm will induce sudden death if surgical repair is not done.