OBJECTIVES: An increased focus on biological age, 'frailty', is important in an ageing population including those undergoing cardiac surgery. None of the existing surgery risk scores European System for Cardiac Operative Risk Evaluation II or Society of Thoracic Surgeons score incorporates frailty. Therefore, there is a need for an additional risk score model including frailty and not simply the chronological age. The aim of this study was to evaluate the impact of frailty assessment on 1-year mortality and morbidity for patients undergoing cardiac surgery. METHODS: A total of 604 patients aged ≥65 years undergoing non-acute cardiac surgery were included in this single-centre prospective observational study. We compared 1-year mortality and morbidity in frail versus non-frail patients. The Comprehensive Assessment of Frailty (CAF) score was used: This is a score of 1-35 determined via minor physical tests. A CAF score ≥11 indicates frailty. RESULTS: The median age was 73 years and 79% were men. Twenty-five percent were deemed frail. Frail patients had four-fold, odds ratios 4.63, 95% confidence interval (CI) 2.21-9.69; P < 0.001 increased 1-year mortality and increased risk of postoperative complications, i.e. surgical wound infections and prolonged hospital length of stay. A univariable Cox proportional hazards regression showed that an increased CAF score was a risk factor of mortality at any time after undergoing cardiac surgery (hazards ratios 1.11, 95% CI 1.07-1.14; P < 0.001). CONCLUSIONS: CAF score identified frail patients undergoing cardiac surgery and was a good predictor of 1-year mortality. CLINICAL TRIAL REGISTRATION NUMBER: NCT02992587.