Idiopathic main pulmonary artery aneurysm is a very rare entity and there are no clear guidelines for optimal treatment. Operative treatment is recommended for patients with a risk of rupture, which is not well defined. We present an unusual case of a 53-year-old woman with an idiopathic main pulmonary artery aneurysm. Our case is asymptomatic, without a causative cardiac lesion and/or pulmonary hypertension; therefore, we did not operate on our patient and she was stable at 22-month follow-up.