We report here a case of a 53-year-old woman with mitochondrial 3243 (A-->G) mutation diabetes. Diabetes mellitus was diagnosed at 39 years of age. At 50 years of age, cardiomyopathy and hypothyroidism were noted. At 53 years of age, the patient was admitted to our hospital for treatment of necrotic ulceration of toes. However, she died of massive heart failure. Pathological examination at autopsy showed a decrease in size, number, and acidophilicity of islets in the pancreas. Immunohistochemical stain revealed a decrease in the beta-cell population to 20.5+/-9.2% and a relative increase in alpha-cell population to 48.6+/-11.5%.