The aim of the study was to assess the impact of atrial fibrillation (AF) on outcome in patients who underwent mitral valve repair (MVRp) for mitral valve prolapse (MVP). Four hundred and forty-three consecutive patients underwent MVRp for organic mitral regurgitation due to MVP. Echocardiography was performed preoperatively and after surgery. Postoperative left ventricular dysfunction (LVD) was defined as left ventricular ejection fraction (LVEF) <50%. Before surgery, 187 patients (42%) had preoperative AF. After surgery, LVEF significantly decreased from 67 ± 9% to 56 ± 10% (p <0.0001). Compared with patients in sinus rhythm (SR), those in AF were significantly older (p <0.0001), had more severe symptoms (p = 0.004), had lower LVEF (p = 0.002), and higher EuroSCORE (p = 0.05). Compared with patients in SR, patients with AF had significantly lower 10-year survival (64 ± 4% vs 83 ± 3%, p = 0.001). On multivariate analysis, preoperative AF was identified as an independent predictor of overall mortality (hazard ratio 1.67; 95% confidence interval 1.15 to 2.42; p = 0.007). At 10 years, patients with paroxysmal AF had lower survival and higher heart failure rate than patients in SR (78 ± 3% vs 66 ± 6%) but had a better outcome compared with those with permanent AF (66 ± 6% vs 53 ± 6%, p = 0.022). Patients with AF had a significantly higher rate of postoperative LVD (23.3% vs 13.4%, p = 0.007). In conclusion, preoperative AF is a predictor of long-term mortality and postoperative LVD after MVRp for MVP. To improve postoperative outcome, surgery in these patients should be performed before onset of AF.