Primary cyst of the diaphragm is a rather uncommon disease. This is a report of a case of diaphragmatic cyst, located in the tendinous portion of the right diaphragm. The patient had no specific symptoms in the respiratory systems, but suffered from gastrointestinal symptoms [indigestion, epigastralgia, and loss of appetite etc.] for 2 months. Accidentally, on a simple chest x-ray examination, a round homogenous mass density was discovered. Tomography showed a well circumscribed parenchymatous mass. So a coin lesion in the medial segment of the right lower lobe was suspected. A thoracotomy was performed. There were no pathological findings in the lung and pleura. A ping-pong ball sized round mass, which was soft, elastic and fluctuated,was noted in the tendinous portion of the diaphragm. It was enucleated completely without diaphragmatic rupture, and diagnosed as a primary diaphragmatic cyst [fibrous-walled] by the histopathologic examination. Postoperative course was uneventful.[KTCS 1978;1:97-101]