Myotonic dystrophy is a well-known hazard of anesthesia for various kinds of surgery. A 47-year-old male who had an increased CTG repeat of approximately 700 copies in the 3'-untranslated region of the myotonic dystrophy protein kinase gene underwent closure of an atrial septal defect under normothermic beating heart. A strong correlation between reduced left ventricular ejection fraction and stroke volume, and the number of CTG repeats, has been reported. Because this correlation is not completely understood, even if the preoperative cardiac function is normal, it is important to check the number of CTG repeats and the patients who have a large number of them should be carefully treated.