BACKGROUND: Atrial arrhythmias (AAs) occur frequently after cardiac surgery. However, most authors do not differentiate postoperative atrial fibrillation (AF) from atrial flutter (AFL). Our study sought to investigate the incidence and predictors of AFL following non-congenital cardiac surgery, and the long-term outcome of this arrhythmia. METHODS AND RESULTS: We conducted an observational, retrospective, single-centre study. Among 821 patients in sinus rhythm (SR) before non-congenital cardiac surgery, AFL occurred in 42 patients (5.1%) and AF in 77 (9.4%). Independent predictors of AAs were older age (odds ratio (OR): 1.06, 95% confidence interval (CI): 1.01-1.11, p = 0.02), low postoperative hemoglobin level (OR: 0.43, CI: 0.29-0.63, p < 0.0001) and long cross-clamp time (OR: 1.02, CI: 1.002-1.037, p = 0.03). Compared to AF patients, those with AFL were younger (67 ± 10 versus 72 ± 8 years, p = 0.005) and less likely to have valve surgery (57% versus 76%, p = 0.03). After conversion to SR, of 41/42 patients with AFL, 5 (12%) developed AFL and 5 (12%) had AF within a mean follow-up of 9.6 ± 8.4 months: only 32% of patients were on long-term antiarrhythmic therapy and 5% had radiofrequency ablation of AFL in the early postoperative period. CONCLUSION: AFL following non-congenital cardiac surgery is not frequent. Compared to patients in AF, those with AFL are younger and less likely to have valve surgery. After conversion of AFL to SR, recurrence rate of AAs is relatively low. Therefore, AFL occurring in this setting does not require systematic catheter ablation in the postoperative period.