Among 21,545 adult patients who underwent consecutive coronary angiography, 16 (0.07%) were found during their coronary arteriography to have a significant isolated stenotic lesion (luminal diameter narrowing of 50% or more) located at the left main coronary artery. The remaining major epicardial coronary arteries and their branches were free of disease. A strong predilection for the isolated lesion to occur at the ostium of the left main artery was found (12 patients). The most common presenting symptom was angina of less than 4 weeks' duration, although one third of the group was asymptomatic. Resting electrocardiograms were normal in 12 patients, while three patients had T wave inversion and another had nonspecific ST-T changes. Eleven patients exhibited severe stenosis, with eight having 70% to 89% stenosis and three having 90% to 95% stenosis. Five patients had 50% to 69% stenosis. No significant differences were found between patients with angina and patients without angina with respect to age, left ventricular end-diastolic pressure, left ventricular ejection fraction, and mean percent stenosis of the obstructive lesion. Despite the severity and the crucial location of the obstructive lesion, most patients with an isolated, significant left main stenosis appear to have a preserved left ventricular ejection fraction, normal wall motion, and no significant alteration of the left ventricular end-diastolic pressure.