A 30-year-old woman with thyrotoxicosis sustained an acute inferolateral-wall myocardial infarction complicated by severe mitral regurgitation and left ventricular wall dysfunction. No obvious predisposing factors were present, and coronary arteriography showed normal patent vessels. Coronary embolism or primary thrombosis in situ of the right coronary artery could explain the arteriographic findings. There is an infrequent association between thyrotoxicosis and acute myocardial infarction, and excess thyroid hormone may influence the extent of the myocardial infarct.