This is a report of a chylous cyst in the mediastinum found in a 61 year old Korean female patient who suffered from mild swallowing difficulty and right chest discomfort. These symptoms had developed 5 years previously when chest P-A view showed thumb-tip sized mass shadow in right mediastinum, just right to ascending aorta. Esophagogram revealed an indentation of midesophagus. Follow up checks showed growing mass shadow which suggested aneurysm of ascending aorta. On admission chest X-ray revealed a well circumscribed infant head sized mass density along the right heart border and middle mediastinal border, and extended near to the middle clavicular line in right chest cavity. Aortogram revealed no relationship between the mass and aorta. Right thoracotomy was performed under impression of mediastinal tumor. There was a large cyst containing 700 cc of milky fluid. There was no connection to the thoracic duct nor adjacent organs. The fluid was analyzed chemically and identified the fluid as chyle. The origin of the isolated chylous cyst was uncertain postoperative course was uneventful.[KTCS 1978;1:102-107]