To elucidate the determinants of the poor prognosis of patients with left main coronary artery disease and to assess the efficacy of diagnostic and therapeutic interventions the angiographic features and clinical course of 58 patients with left main coronary artery disease studied between September 1967 and June 1974 were analyzed. Eighty-three coronary arteriograms were obtained in these 58 patients using the Judkins technique; there were no immediate complications although one patient died 3 days after study. Previously cited predictors of left main coronary artery, unstable or nonexertional angina and marked S-T segment depression with exercise were found in a minority of patients; thus, the presence of the disease could not reliably be predicted before arteriographic study. Coexisting disease was found in either two or three other coronary arteries in 46 of 58 patients; only 2 patients had isolated left main coronary artery disease. Because the criteria for operability have changed in recent years, current criteria without knowledge of the treatment actually given or its outcome. The condition of 10 of 58 patients was judged inoperable in retrospect because of severe coexisting distal coronary artery disease (8 patients) or ventricular dysfunction (2 patients). Of 19 patients whose condition was judged operable in retrospect but who were treated without surgery, 9 died, 8 within 18 months; 10 have survived 12 to 83 months. Another 27 patients with a condition judged operable in retrospect had received saphenous vein bypass grafts. In this group, there were four operative and three late deaths. The severity of angina decreased in survivors treated surgically but was unchanged in survivors treated without surgery. The improvement in survival rates of surgically treated patients was not statistically significant. The data indicate that coronary arteriography can be performed at low risk with the Judkins technique even though preangiographic prediction of left main coronary artery disease is unreliable. Coexisting disease in oter major coronary arteries is an important determinant of the poor prognosis of patients with left main coronary artery disease and precludes surgery in 13 percent. Isolated left main coronary artery disease is uncommon. Surgical therapy relieves symptoms more effectively than nonsurgical therapy.