The case is reported of a 69-year-old woman who for the last 2 years of her life had severe muscle weakness (with elevated muscular enzymes), thyroid abnormalities (autoantibodies and increase in TSH) and a slowly growing mass in the anterior mediastinum. She died in cardiogenic shock. The diagnosis of "nodular polymyositis" (painful nodules palpable in the muscles) with heart involvement was strongly suspected. The thyroid abnormalities were considered to be "biological thyroiditis" i.e. a "disease" frequent in normal females (5-10%) and, after differential diagnosis of anterior mediastinal masses, the diagnosis of thymoma was adopted in view of its frequent association with autoimmune diseases. Moreover, in rare cases of thymoma associated with polymyositis, histology has disclosed "giant cell polymyositis". Autopsy revealed giant cell polymyositis, a thymoma and lymphocytic infiltration of the thyroid.