A 34-year-old female IDDM patient complained of chest oppression in hypoglycemic episodes and electrocardiograms revealed reversible ischemic changes occurring concomitantly with hypoglycemia. The ECG changes improved and the chest oppression disappeared following increasing blood glucose level by glucose intake. Master's double load test and treadmill load test were positive for ischemic changes. Radioisotopic myocardial scintigraphy by thallium and BMIPP did not show any filling defects and coronary angiography revealed no remarked stenosis in the coronary arteries. She had no mitochondrial tRNA(Leu) (A-->G) gene mutation at nucleotide position 3243, but both the patient and her mother had a G-to-A transition within the replication origin of the light strand at nucleotide position 5744 of the mitochondrial gene. As the patient's maternal family had no history of ischemic heart disease, it is not clear whether mitochondrial gene mutation at nucleotide position 5744 reflects the occurrence of cardiac ischemia. Some disorders of microcirculation in capillary vessels in cardiac muscles may occur in such patients.