The clinical and angiographic findings in 10 patients with anomalous origin of the left main (LM) coronary artery from the right sinus of Valsalva are presented. The initial course of the anomalous artery was between the aorta and the pulmonary trunk in 6 patients (5 men and 1 woman), and posterior to the aorta in 4 patients (3 women and 1 man). The best coronary angiographic projection to determine the course of the LM coronary artery in relation to the aorta and pulmonary trunk was the right anterior oblique and lateral projections, with a catheter in the pulmonary trunk as a guide. In the right anterior oblique projection left ventriculogram, an increased density at the base of the aorta was seen in all patients with posterior-to-aorta course of the anomalous artery. The blood supply to the first portion of the ventricular septum is provided by 1 or 2 descending septal branches from the anomalous LM artery, when the anomalous vessel is coursing between the aorta and pulmonary trunk. When the anomalous LM courses posteriorly to the aorta, it does not provide any septal branches. In patients with this anomaly, the blood supply to the first portion of the septum is usually provided by descending septal branches originating from the right sinus of Valsalva or from the first portion of the right coronary artery. Six of the 10 patients had severe obstructive coronary artery disease. Four of the 6 patients had significant LM coronary artery disease.