Abstract： Objective To determine the influence of preoperative atrial fibrillation （AF） on midterm and longterm clinical outcomes of patients after mitral valve replacement （MVR）. Methods We retrospectively analyzed clinical data of 1 029 patients who underwent MVR with or without tricuspid valve repair in Changhai Hospital， Second Military Medical University， from January 2000 to December 2005. According to the exclusion criteria， 621 patients were selected and divided into two groups depending on presence of preoperative AF. Those 395 patients with preoperative AF belonged to the AF group， including 134 males and 261 females with their average age of 51.1±11.5 years. Those 226 patients with preoperative sinus rhythm （SR） were in the SR group， including 82 males and 144 females with their average age of 48.2±14.1 years. Early postoperative outcomes， midterm and longterm mortality and morbidity of the two groups were compared. Results During 10 years of follow-up， there was no statistical difference in early postoperative mortality and morbidity between the two groups， but the incidence of late thromboembolism was significantly higher in AF group than that in SR group ［0.9‰ （31 patients/33 984 patient-months） vs. 0.4‰ （9 patients/21 151 patient-months）， χ2=4.26， P=0.039］. Ten-year survival rate in patients in AF group was significantly lower than that in SR group （83.2% vs. 92.7%， χ2=10.26， P=0.002）. Multivariate analysis identified preoperative AF ［HR=2.878， 95% CI （1.166，4.129）］， low left ventricular ejection fraction ［HR=0.948， 95% CI （0.917，0.981）］ ， and old age ［HR=1.073， 95% CI （1.038，1.109）］ as independent risk factors for late mortality after MVR. Apart from its influence on patient survival rate and incidence of thromboembolism， preoperative AF also had an adverse effect on left ventricular function， right ventricular function and tricuspid regurgitation. Conclusion AF is an independent risk factor for poor prognosis after MVR. Prognosis after MVR might be improved if surgery could be performed early when patients have predictive signs of AF such as multiple premature atrial contractions or left atrium enlargement.