The authors report a case of pleural tumor discovered in case of thoracic pain in a 72 year old woman. This lesion, discovered on chest X-Ray, developed in the left costo-vertebral groove and was seen as a homogeneous opacity, multilobate, well-defined. CT confirmed the pleural origin, showed a thickening of posterior pleural, a polylobate tumor well-defined on its lung side, non-invasive and of tissue density. Histopathological examination of the operative specimen showed it to be a thymoma with an exceptional localization.