A 67-year-old woman presenting with a 9-month history of polymyositis, suddenly worsened her clinical state and died. An autopsy was performed, finding a tumor measuring 8 x 6 x 1 cm, located in the left pulmonary hilum. The thyroid gland, of normal volume, was firm. The histological analysis of the tumor, the thyroid gland and several skeletal muscles led to the following diagnoses: thymic carcinoma, Hashimoto's thyroiditis and polymyositis. The aim of the discussion is this thymic carcinoma, the originality of which are firstly its ectopic location, secondly its association with immune diseases usually described during the course of thymomas rather than thymic carcinomas.