Twenty-five patients(6.3% of 400 cases with significant coronary disease) with 50 percent or greater diameter obstruction in the left main coronary artery(Group 1) were compared with 187 patients with at least two vessel disease without left main involvement (Group 2) in clinical, arteriographic and hemodynamic findings. The ratio of male to female was 4:1 in Group 1 and 4.8:1 in Group 2, and the average was 51.0+/-11.6 and 55.6+/-8.6 years respectively. Twenty-two patients(88%)had moderate to server angina pectoris important risk factors, cigarette smoking was present in 10 patients(40%), hypertension in 7 (28%), diabetes mellitus in 7 (28%), and hypercholesterolemia(over 220 mg%) was in 4 (16%) out of 25 patients(Group 1). There were no difference between patients with Group1 and Group 2 in sex, clinical manifestation, duration of chest pain, treadmill test, major risk factors and the average resting left ventricular end-diastolic pressure. But the average age of Group 1 was slightly lower than that of Group 2(P<0.05). Image intensification fluoroscopy revealed clacification in the left main coronary artery in 5 patients. Sixteen patients(64%)had significant obstructive disease in the other coronary arteries. Contraction abnormalities in the left ventricullgram were present in 7(28%). The left ventricular end-diastolic pressure wea greater than 12mmHg in 13 of the 23 patients(56.5%). The average resting left ventricular end-diastolic pressure was 12.0+/-5.2mmHg. No significant comlplications were associated with cardiac catheterization. Three of 11 patients who underwent aocoronary bypass surgery died. One patients underwent PTCA, and the remaining 13 patients were treated medically.