A 57-year-old man with an idiopathic aneurysm of the left pulmonary artery is presented. The aneurysm was stable over many decades. No such long-term follow-up of an idiopathic pulmonary aneurysm has been described previously. After 39 years there is no evidence of heart failure or pulmonary hypertension. Because corrective surgery has a variable outcome, and prognosis is suggested to be good in the idiopathic form of pulmonary aneurysm, no surgical correction was proposed.