Giant cell myocarditis is a rare disease of unknown etiology, which develops as a myocardial isolated affection or associated to different diseases. Its characteristics are such as necrosis, inflammation and giant cell presence in the myocardium. We present the case of a woman who suffered of giant cell myocarditis, thymoma, myasthenia gravis, chronic lymphocytic thyroiditis, giant cell myositis, granulomatous infiltration in the lymph nodes of the hilus of the lung and hypogammaglobulinemia; multiple association that we have not found in any published medical paper and that suggest the autoimmune origin of this illness. The cardiovascular symptoms and the associated diseases are revised, and we discussed the diagnostic and therapeutic topics, pointing out the necessity to take it into account for any patient with thymoma or myasthenia gravis developing to heart failure or arrhythmias.