Background and aim of the study: 30% to 40% of patients (pts) have chronic AF before mitral valve operation. In the most pts AF will persist after procedure. In this study a radiofrequency (RF) ablation was performed. Material and methods: two groups were analyzed: RF group–50 pts with mitral valve replacement and ablation, Control group–50 pts with mitral valve replacement. The Student’s t-test and Chi2 test were used. Results: the groups were compared according to: age, gender, EF, LA size, the grade of heart insufficiency , the type of mitral valve’s dysfunction and the history of embolic events. The differencies were insignificant. Aortic crossclamp time was longer in RF group (p<0.001). The pacemaker was implanted in 4 pts from RF group and in 1 patient from the second one. Cardioversion was performed less frequently and with better results in RF group (p=0,002). After one year the average size of LA was lower in pts with sinus rhythm than in pts with AF (p=0.042). EF was higher (p=0.016). LA size > 6 cm 5-fold at the discharge and 9.3-fold after one year increased the risk of ablation failure than the small one (LA<5cm). NYHA class IV 36-fold at the discharge and 37-fold after 1 year increased the risk of AF preservation than NYHA class II. Conclusions: 1. RF ablation statistically significantly restores sinus rhythm 2. LA size>6 cm and NYHA class IV are the most important risk factors for ablation failure.