A 25 year old man with no previous history was admitted for a recurrent right sided encysted pleural effusion. Thoracocentesis revealed an exudative lipase-rich-fluid. Because of the persistence of abundant pleural fluid after several thoracocentesis a diagnostic mediastinotomy was performed. This revealed a necrotic cystic mediastinal tumor composed of well differentiated pancreatic tissue, nervous tissue, digestive and respiratory glands, and a dermoid mass with hair and sebacious glands. This led to a diagnosis of benign, mature mediastinal teratoma. We suggest that the chronic pleural effusion was maintained by a fistula from the tumour into the pleural cavity and the secretion of proteolytic enzymes by the well-differentiated pancreatic tissue.