We measured the coronary flow reserve in a 13-year-old girl with the rare anomaly of an absent left circumflex coronary artery. Although the coronary flow volume of the right coronary artery was at the same level as that of the left anterior descending coronary artery, the coronary flow reserve of the patient's right coronary artery was depressed without stenotic lesion, and it was less than the level of -2 standard deviation (SD) of the right coronary artery preponderance cases.