A 56-year-old male with the Wolff-Parkinson-White syndrome was suspected of having suffered a myocardial infarction following attacks of chest pain. Serial measurements of serum creatine phosphokinase and the electrocardiographic findings after ajmaline loading virtually excluded the possibility of myocardial infarction. Paroxysmal tachycardia was not noted on 24 Holler electrocardiographic monitoring. Both the left and right coronary arteries were found by selective coronary angiography to originate from the left sinus of Valsalva, but neither of the arteries showed organic stenoses. However, myocardial²⁰¹ Tl scintigraphy after exercise indicated poor uptake in the high anterolateral wall, and at the apex and septum near the anterior wall, which suggested the absence ofischaemia in the area supplied by the right coronary artery. We speculate that a left coronary artery steal phenomenon was the cause of the ischaemia after exercise in this patient.