In this report, cardiac abnormalities in 2 diabetic patients with a mutation in the mitochondrial tRNA(Leu)(UUR)gene are described. The 2 patients exhibited left ventricular dilation and diffuse decreased wall motion. Cardiac catheterization studies revealed normal coronary angiography. In both patients, light microscopic examination of hematoxylin and eosin-stained sections revealed myocyte hypertrophy and perinuclear vacuolization and electron microscopy revealed an increase in the number and size of mitochondria and abnormal configuration of cristae. These findings suggest that mutations in mitochondrial DNA, such as an A-->G mutation at position 3243, should be considered as a cause not only of diabetes mellitus but also of cardiac involvement in patients with diabetes mellitus.