This report presents the case of a 47-year-old female patient with fulminant type 1 diabetes mellitus and myocarditis. Following a high fever, nausea, vomiting and diarrhea, diabetic ketoacidosis occurred and she was transferred to the hospital. The plasma glucose level was 63.6mmol/L and HbA1c was 7.0%. C-peptide was undetectable in her plasma. Blood gas analysis showed a pH of 6.99. Antibodies to glutamic acid decarboxylase nor insulinoma associated antigen-2 were not detected. She was diagnosed to have fulminant type 1 diabetes mellitus. Her electrocardiogram showed diffuse ST-segment elevations on the second day of admission, along with a positive troponin test. However coronary angiography revealed neither occlusion nor stenosis of the cardiac arteries. An endomyocardial biopsy revealed hypertrophic cardiomyocytes with a disarrangement of myofibers and the focal accumulation of mononuclear cells in the stroma, thus suggesting myocarditis or mild myocarditic change. Viruses are an important cause of myocarditis and the preceding flu-like symptoms indicate the association of viral infection with myocarditis in this case. The mechanisms by which fulminant type 1 diabetes mellitus occurs is still uncertain, but the presence of islet injury accompanied by myocardial inflammation in the current case suggested that a viral infection accounted for the onset of this type of diabetes.