The estimation of frequency of occurence of atrial fibrillation in patients with mitral valvular diseases was made before operation and at the early postoperative period. A logistic regressive analysis has shown that the appearance of atrial fibrillation in long-term periods is influenced by 4 independent preoperative factors: preoperative atrial fibrillation, age, anterior-posterior size of the left atrium, diagnosis of rheumatism. The dependence of the appearance of atrial fibrillation on the preoperative status of the patient was detected. A statistical model having the prognostic value 89.6% has been constructed. The model is good in use, reliable and can be effectively used in everyday clinical practice in order to determine the probability of the appearance of atrial fibrillation at the postoperative period of the concrete patient and decision on necessity of surgical ablation.