Background The results of SYNTAX trial have been reported based on "corelab" calculated SS (cSS). It has been shown that reproducibility of SS is better among the core laboratory technicians than interventional cardiologists. Thus, the prognostic value and clinical implication of the "site" SYNTAX SS (sSS) remain unknown. Objectives The study sought to evaluate the prognostic value and clinical implication of the sSS after percutaneous coronary intervention (PCI) or coronary artery bypass graft (CABG) surgery in the randomized SYNTAX trial. Methods The sSS was calculated by the site investigators before randomization in the SYNTAX trial. New tertiles based on the sSS were defined with low (0 to 19), intermediate (20 to 27), and high (≥28) scores. The clinical endpoints were compared between PCI and CABG by Kaplan-Meier estimates, log-rank comparison, and Cox regression analyses using the new tertiles. The sSS-based SS II was calculated and its predictive performance was evaluated. Results The mean difference in cSS and sSS is 3.8 ± 11.2, with a mean absolute difference of 8.9 ± 7.8. In the overall cohort, using sSS there was a higher incidence of major adverse cardiac and cerebrovascular events (MACCE) at 5-year follow-up in the PCI group for low (31.9% vs. 24.5%; p = 0.054), intermediate (39.5% vs. 29.5%; p = 0.019), and high (43.0% vs. 31.4%; p = 0.003) tertiles, compared with the CABG group. Similarly, in the 3-vessel disease subgroup, 5-year MACCE rates were higher in PCI group in all tertiles. Conversely, in the left main subgroup, MACCE rates were similar for PCI and CABG groups in all tertiles. The sSS-based SS II (c-index: 0.736) had predictive performance similar to the cSS-based SS II (c-index: 0.744), with net reclassification index of -0.0062 (p = 0.79). Conclusions Appropriate training and unbiased assessment are needed when using SS in clinical decision making. sSS and tertiles based on sSS showed poor discrimination among low, intermediate, and high-risk groups. However, combining clinical factors with sSS retained the predictive performance of SS II. (SYNTAX Study: TAXUS Drug-Eluting Stent Versus Coronary Artery Bypass Surgery for the Treatment of Narrowed Arteries; NCT00114972) © 014 by the American College of Cardiology Foundation. © 2014 by the American College of Cardiology.