It is accepted universally that the treatment of critical left main coronary artery (LMCA) stenosis is surgical revascularization. This study was designed to evaluate critical LMCA stenosis as a risk factor in coronary artery bypass surgery. We compared the surgical results of 760 patients with critical LMCA disease, including 58 cases who were operated under emergency conditions (LMCA-em) and 702 patients who were operated electively (LMCA-el), with randomly chosen 707 coronary bypass patients (CONT-el) without LMCA disease, but who had double- or triple-vessel disease. Another group of patients (n = 99) who were operated on under emergency conditions (CONT-em) but without LMCA disease were also compared with the corresponding groups. The mortality of LMCA-em group and CONT-em group was markedly higher from the other two groups. Univariate analysis revealed that female gender, older age, presence of diabetes mellitus, poor left ventricular function, and the presence of unstable angina were major risk factors for fatal outcome in LMCA-el and CONT-el groups. Age was also a risk factor in LMCA-em group, as well as unstable angina pectoris. The coexistence of critical right coronary artery disease did not affect the early outcome in both groups with LMCA lesions. In the multivariate analysis, critical LMCA disease was not a risk factor for mortality. Logistic regression analysis revealed diabetes [odds ratio (OR): 3.66], poor left ventricular function (higher left ventricle end-diastolic pressure, OR: 1.08), and emergent operations (OR: 5.09) were risk factors for early mortality. Patients with LMCA disease should have surgery promptly for favorable results, because emergency conditions have higher mortality rates.