Teratomas account for 8-13% of all mediastinal tumors. A 27-year-old patient presented with chest pain and dyspnea of prolonged evolution. A chest X-ray revealed near total opacification of the right hemithorax. On a tomography scan of the chest, a collection of heterogeneous fluid, with irregular borders and 10.1 x 11.7 cm in size, was seen in the pleura of the lower two-thirds of the right hemithorax but was not encroaching on any of the adjacent structures. Based on the hypothesis that these findings represented a benign mediastinal teratoma, an exploratory thoracotomy was performed, during which such a teratoma was found and completely excised. The post-operative evolution was favorable. The atypical presentation and considerable growth of the tumor hindered the pre-operative diagnosis.

Keywords: Teratoma; Mediastinal neoplasms; Pleural effusion; Radiography thoracic