Background: A comprehensive angiographic scoring system of the SYNTAX-score was developed to help physicians predict long-term outcome or select a revascularization strategy. However, its validity has not been systemically evaluated for patients with unprotected left main coronary artery (ULMCA) stenosis. Methods: From 2240 patients who received either stenting or coronary artery bypass graft (CABG) surgery for ULMCA stenosis enrolled in the MAIN-COMPARE registry, the SYNTAX-score was analyzed in angiographic core laboratory for 1580 (70.5%) patients including 819 receiving stenting and 761 receiving CABG. The outcomes of interest were the major adverse vascular events (MACE) including death, Q-wave myocardial infarction, or stroke, and major adverse cardiac and cerebrovascular events (MACCE). The patients were classified into the low- (<22), intermediate- (>22 and <33), and high- (>33) score groups. Results: The score was not normally distributed (p<0.001) with median of 30.0 and interquartile range (IQR) of 19.0 to 40.5, which was higher in CABG (37.5; IQR, 29.0 to 47.5) than stenting (23.0; IQR, 14.0 to 31.5) group (p<0.001). During 3 years, high-score group had higher incidence of MACE for overall patients (5.7%, 6.8%, and 12.1%, respectively; p<0.001) and stenting group (4.7%, 7.0%, and 12.8%, respectively; p=0.002), but not for CABG group (10.5%, 6.6%, and 11.8%, respectively; p=0.18). However, there was no significant association between the 3 SYNTAX classifications and the MACCE rate in overall patients (p=0.52), stenting group (p=0.062), or CABG group (p=0.44). In Cox model, a significant interaction between treatment type and SYNTAX groups was not observed in the risk of MACE or MACCE. Conclusions: A SYNTAX score appears to be associated with the risk of MACE after stenting for ULMCA stenosis. However, the differential effectiveness of stenting versus CABG in the risk of MACE or MACCE seems to be independent of the SYNTAX-score classifications.