Objective: This article presents a study of postoperative atrial fibrillation (AF) and its long-term effects on mortality and heart rhythm. Methods: The study cohort consisted of 571 patients with no history of AF who underwent primary aortocoronary bypass surgery from 1999 to 2000. Postoperative AF occurred in 165/571 patients (28.9%). After a median follow-up of 6 years, questionnaires were obtained from 91.6% of surviving patients and an electrocardiogram (ECG) from 88.6% of all patients. Data from hospitalisations due to arrhythmia or stroke during follow-up were analysed. The causes of death were obtained for deceased patients. Results: In postoperative AF patients, 25.4% had atrial fibrillation at follow-up compared with 3.6% of patients with no AF at surgery (p ≪ 0.001). An episode of postoperative AF was the strongest independent risk factor for development of late AF, with an adjusted risk ratio of 8.31 (95% confidence interval (CI) 4.20–16.43). Mortality was 29.7% (49 deaths/165 patients) in the AF group and 14.8% (60 deaths/406 patients) in the non-AF group (p ≪ 0.001). Death due to cerebral ischaemia was more common in the postoperative AF group (4.2% vs 0.2%, p ≪ 0.001), as was death due to myocardial infarction (6.7% vs 3.0%, p = 0.041). Postoperative AF was an age-independent risk factor for late mortality, with an adjusted hazard ratio of 1.57 (95% CI 1.05–2.34). Conclusions: Postoperative AF patients have an eightfold increased risk of developing AF in the future, and a doubled long-term cardiovascular mortality.