The SYNTAX score II (SSII) was developed from the SYNTAX trial to predict the 4-year all-cause mortality after left main or multivessel disease revascularization and to facilitate the decision-making process. The SSII provides the following treatment recommendations: (i) coronary artery bypass grafting (CABG) or percutaneous coronary intervention (PCI) (equipoise risk), (ii) CABG preferred (excessive risk for PCI) or (iii) PCI preferred (excessive risk for CABG). We sought to externally validate SSII and to investigate the impact of not abiding by the SSII recommendations in the randomized EXCEL trial of PCI versus CABG for left main disease.The calibration plot of predicted versus observed 4-year mortality was constructed from individual values of SSII in EXCEL. To assess overestimation versus underestimation of predicted mortality risk, an optimal fit regression line with slope and intercept was determined. Prospective treatment recommendations based on SSII were compared with actual treatments and all-cause mortality at 4 years.SSII variables were available from EXCEL trial in 1807/1905 (95%) patients. For the entire cohort, discrimination was possibly helpful (C statistic = 0.670). SSII-predicted all-cause mortality at 4 years overestimated the observed mortality, particularly in the highest-risk percentiles, as confirmed by the fit regression line [intercept 2.37 (1.51-3.24), P = 0.003; slope 0.67 (0.61-0.74), P < 0.001]. When the SSII-recommended treatment was CABG, randomized EXCEL patients treated with PCI had a trend towards higher mortality compared with those treated with CABG (14.1% vs 5.3%, P = 0.07) in the as-treat population. In the intention-to-treat population, patients randomized to PCI had higher mortality compared with those randomized to CABG (15.1% vs 4.1%, P = 0.02), when SSII recommended CABG.In the EXCEL trial of patients with left main disease, the SSII-predicted 4-year mortality overestimated the 4-year observed mortality with a possibly helpful discrimination. Non-compliance with SSII CABG treatment recommendations (i.e. randomized to PCI) was associated with higher 4-year all-cause mortality.