The SYNTAX score has been designed to better anticipate the risks of percutaneous or surgical revascularisation, taking into account the functional impact of the coronary circulation with all its anatomic components including the presence of bifurcations, total occlusions, thrombus, calcification, and small vessels. The purpose of this paper is to describe the baseline assessment of the SYNTAX score in the Syntax randomised trial, the corelab reproducibility, the potential difference in score assessment between the investigator and the corelab, and to ascertain the impact on one-year outcome after either percutaneous coronary intervention (PCI) or coronary artery bypass surgery (CABG) in patients with complex coronary artery disease.To assess the reliability of Syntax scoring, 100 diagnostic angiograms from the Syntax trial were randomly selected and assessed independently by two observers. Intra-observer variability was assessed by analysing 91 sets of angiograms after an interval of at least eight weeks by one of the observers. Clinical outcomes in the randomised cohort of the Syntax trial up to one year are presented with stratification by tertile group of the SYNTAX score. The weighted kappa value for the inter-observer reproducibility on the global score was 0.45, while the intra-observer weighted kappa value was 0.59. The SYNTAX score as calculated by investigators consistently underscored the corelab score by 3.4 points. When the Syntax randomised cohort was stratified by tertiles of the SYNTAX score, there were similar or non-significantly different MACCE rates in those with low or intermediate scores; however in the top tertile the MACCE rate was greater in those receiving PCI compared to CABG.The SYNTAX score is a visual coronary score with an acceptable corelab reproducibility that has an impact on the one-year outcome of those having PCI, whereas it has no effect on the one-year outcome following surgical revascularisation. The SYNTAX score tool is likely to be useful in a wide range of patients with complex coronary disease.