The prognosis of 141 patients with obstructive left main coronary artery disease was studied. The patients were followed up for 5 to 10 years from the date of coronary arteriography. The 5 year cardiac mortality rate was 51 percent. During the first 3 years after diagnosis, the mortality rate was especially high, 21.9, 34.7 and 43 percent, respectively. Among patients who died in the 1st year after diagnosis, evidence of left ventricular hypertrophy or conduction delay in the electrocardiogram and multiple vessel involvement indicated poor prognosis. Severe angina pectoris, evidence of congestive heart failure and generalized decrease in left ventricular contractility increased the 5 year mortality rate. More than 97 percent of the total patient population had additional lesions elsewhere in the coronary arteries.