A 38-year-old man had suffered from general fatigue, mild weakness of proximal muscles, and dry cough in November, 2000. Serum levels of muscle enzymes were elevated. Computed tomography of the chest revealed reticular appearance in the bilateral dorsal lung areas. He did not show any improvement, therefore he was referred to our hospital in April, 2001. He was diagnosed as mild polymyositis with mild interstitial pneumonia. He was treated intravenously with methylprednisolone pulse therapy. During the pulse therapy serum level of creatine kinase was decreased, but he died because of acute pump failure of the heart. The cause of the heart failure could be an exacerbation of chronic myocarditis associated with polymyositis and it was confirmed by autopsy findings. When a patient with mild polymyositis complains of general fatigue, myocarditis should be carefully evaluated because of the high risk of death.