Three patients with the rare anomaly of congenital absence of the ostium of the left main coronary artery are presented. In two of the patients, aged 50 and 52 respectively, the diagnosis was established during selective coronary cineangiography for a severe anginal syndrome. The third patient, a 16-year-old girl, underwent cardiac catheterization for investigation of a congenital heart malformation, when a single right coronary artery was demonstrated with absence of the main coronary artery ostium. Two patients underwent successful aortocoronary bypass grafting. In view of the occurrence of sudden death and massive myocardial infarction in adult patients shown to have severe or complete obstruction of the left main coronary artery, it is suggested that adult patients with this condition, who require open-heart surgery for any other cardiac disorder, should undergo aortocoronary bypass grafting concurrently even prior to the development of anginal symptoms. Children shown to have this anomaly should be subjected to long-term follow-up and have an aortocoronary bypass graft performed when symptoms of coronary insufficiency develop.