Cystic intrathoracic lesions of foregut origin are now well recognized and account for approximately 10% of lesions presenting as mediastinal tumors. The terminology used to describe mediastinal endodermal cysts has been confused and sometimes ambiguous. The embryological derivation of these lesions has been the cause of much speculation. It Is suggested that these lesions should be classified Into three main categories based on embryology bronchogenic cyst[resulting from a defect of lung budding], Intramural esophageal cyst[true duplication], and enteric cyst[resulting from the split notochord syndrome]. This communication describes a 26 year old man with intramural esophageal cyst who was diagnosed as posterior medlastlnai tumor preoperatively and cured with extirpation of the cyst.[KTCS 1981;1:95-97]