The aim of this study was to assess the impact of frailty on the outcome after coronary artery bypass grafting (CABG) and whether it may improve the predictive ability of European System for Cardiac Operative Risk Evaluation (EuroSCORE II). The Clinical Frailty Scale (CFS) was assessed preoperatively in patients undergoing isolated CABG from the multicentre E-CABG registry, and patients were stratified into 3 classes: scores 1–2, scores 3–4 and scores 5–7. Of the 6156 patients enrolled, 39.2% had CFS scores 1–2, 57.6% scores 3–4, and 3.2% scores 5–7. Logistic regression adjusted for multiple covariates showed that the CFS was an independent predictor of hospital/30-day mortality [CFS scores 3–4, odds ratio (OR) 3.95, 95% confidence interval (CI) 2.19–7.14; CFS scores 5–7, OR 5.90, 95% CI 2.67–13.05] and resulted in an Integrated Improvement Index of 1.3 (P < 0.001) and a Net Reclassification Index of 55.6 (P < 0.001) for prediction of hospital/30-day mortality. Adding the CFS classes to EuroSCORE II resulted in an Integrated Improvement Index of 0.9 (P < 0.001) and Net Reclassification Index of 59.6 (P < 0.001) for prediction of hospital/30-day mortality with a significantly larger area under the receiver operating characteristics curve (0.809 vs 0.781, P = 0.028). The CFS was an independent predictor of mid-term mortality [CFS scores 3–4, hazard ratio (HR) 2.05, 95% CI 1.43–2.85; CFS scores 5–7, HR 3.05, 95% CI 1.83–5.06]. The CFS predicted early- and mid-term mortality in patients undergoing isolated CABG. Further studies are needed to evaluate whether frailty may improve the estimation of the operative risk of patients undergoing adult cardiac surgery. NCT02319083.