The authors report on a 76-year-old woman presenting with malignant thymoma associated with myasthenia, erythroblastopenia, myositis and giant cell myocarditis. Blood examination showed antinuclear antibodies, anti-platelet antibodies and LE cells. Only one similar case and 22 cases of thymoma associated with giant cell myocarditis have been published. It seems probable that thymomas plays a part in the pathogenesis of giant cell myocarditis.