Objectives. Typically, patients referred to cardiac surgery are aged. Because EuroSCORE tend to overestimate and STS tend to underestimate the risk of mortality after cardiac surgery, frailty has become interesting as a potential predictor for mortality after cardiac surgery. Therefore, we conducted a study to identify the number of frail patients undergoing cardiac surgery and describe the risk of short-term complications and mortality. Design. In a prospective observational study, we have compared the surgical outcome in frail versus non-frail patients. Patients aged > 65 years and undergoing non-acute cardiac surgery were included. Frailty was assessed using the comprehensive assessment of frailty (CAF) score. The CAF evaluates the patient’s physical condition through performing physical tests. Results. 604 patients included, 477 were men and the median age was 73 years (range, 65–90). Twenty-five percent were deemed frail. Frail patients had a four times higher 30-day mortality. Furthermore, frail patients had higher postoperative complication rates of atrial fibrillation, prolonged ventilation, re-operations, renal failure, transfusion requirements, and increased length of stay. Patients who died within 30 days had a significantly higher CAF score than those who survived (p =.039). Based on ROC curves, the area under the curve (AUC) for CAF score was 0.700, EuroSCORE 0.664 and STS score 0.748. Conclusion. Frailty is common in patients undergoing cardiac surgery and carries increased risk of 30-day mortality and postoperative complications. The AUC indicates similar prediction of mortality for CAF score compared to the existing risk scores. Clinical Trials Registration ID: NCT02992587.