Background: Delirium or acute confusion is a temporary mental disorder that occurs frequently among hospitalized elderly patients. Patients who undergo cardiac surgery have an increased risk of delirium, which is associated with many negative consequences. Therefore, prevention or early recognition of delirium is essential. Methods: In this observational study, a risk checklist for delirium was used during the preoperative outpatient screening in 112 patients who underwent elective cardiac surgery. The Delirium Observation Screening (DOS) scale was used before and after surgery to assess whether delirium had developed in patients. The psychiatrist was consulted to confirm or refute the diagnosis delirium. Results: The incidence of delirium after cardiac surgery was 21%, and the mean duration of delirium was 2.5 days. The time to discharge was 11 days longer for patients with delirium. The delirium risk checklist could accurately predict postoperative delirium in patients who underwent elective cardiac surgery based on a disturbance in the electrolytes sodium and potassium and on EuroSCORE (European System for Cardiac Operative Risk Evaluation). When using a probability of delirium of 50%, the sensitivity of the risk checklist was 25.0% and specificity was 95.5%. The predictive value of a positive test was 60.0%, and the predictive value of a negative test was 82.4%. The area under the receiver-operating characteristic curve was 0.75. Conclusions: With the risk checklist for delirium, patients at an increased risk of delirium after elective cardiac surgery can be identified.