We report 5 cases of isolated ostial stenosis of the left main coronary vessel. Isolated ostial stenosis occurs preferentially in young or middle aged women for whom coronary insufficiency is usually not a serious threat. Patients with this type of lesion have characteristically severe angina of relatively recent onset. The condition may be difficult to diagnose at angiography, but a fall in pressure when the tip of the catheter enters the coronary lumen beyond the stenosis, a lack of reflux of the contrast medium into the sinus of Valsalva during intracoronary injection and its persistence in the coronary vessel should alert the investigator. A pathological study of 3 cases revealed typical atheromatous lesions in 2 patients (with extension of an aortic plaque to the left coronary ostium in one, and atheroma localized on the ostium in the other) and a purely fibrous lesions in a patient who had undergone thoracic radiotherapy 5 years previously. Although relatively rare, stenosis must be diagnosed in view of its sombre spontaneous prognosis (one patient died 3 days after coronary arteriography), of the risk of underestimating its frequency, and of the hazards of selective coronary catheterization in such patients (one of our patients died 15 minutes after coronary exploration).