The benign tumor of smooth muscle, the leiomyoma, may arise anywhere in the body, in such tissues as the muscularis of the gut and the media of blood vessels, although by far its most common location is the uterus. Indeed, the mediastinal leiomyomas are extremely rare, especially when one excludes those arising in the esophagus. No dogmatic statements can be made with regard to age, sex distribution, symptoms, or clinical course because of the extreme paucity of cases available for evaluation. Although the majority of the reported cases are in the posterior mediastinum, thus suggesting esophageal origin. We have experienced a right inferior mediastinal leiomyoma in 36 years old housewife. She complained hemoptysis and right lower chest pain associated with intermittent low-graded fever and chillness. Routine X-ray film revealed a large irregular cystic mass density in the right lower thoracic cavity. On exploratory thoracotomy, a huge round yellow-gray colored solid mass, measured about 2.5kg in weight, was located in the right inferior mediastinum just above right hemidiaphragm, medially. The tumor mass was removed, being proved to be leiomyoma on histopathologic examination. Her postoperative course was uneventful and discharged without complication.[KTCS 1974;1:117-122]