Idiopathic dilatation of the pulmonary artery is an uncommon, but not rare, congenital anomaly that is diagnosed by exclusion and generally has a benign clinical course. A left hilar mass on the chest x-ray film is the major presentation. The authors present a case of idiopathic dilatation of the pulmonary artery with posttraumatic chest pain clinically. After serial examinations, idiopathic dilatation of the pulmonary artery was diagnosed. They discuss the phenomenon of transpulmonary valvular pressure gradient and noninvasive tools for long-term follow-up, including transesophageal echocardiography and magnetic resonance imaging.