Introduction. Atrial fibrillation (AF) and obstructive sleep apnea (OSA) are common in the population and share common risk factors. The prevalence of OSA in AF is established, but the incidence of AF in patients with OSA is not studied.Purpose. To study the incidence of atrial fibrillation and atrial remodeling in patients with varying severity of obstructive sleep apnea syndrome.Materials and methods. Retrospective analysis of 216 medical records of patients hospitalized in a therapeutic hospital in whom screening respiratory monitoring revealed OSA.Results. Atrial fibrillation was diagnosed in 62/216 patients (28,7 %) with OSA. AF was more often observed in patients with severe OSA – in 27/76 (35,5 %) compared to those examined with a mild degree – 16/78 (20,5 %) (p=0,043). Permanent AF was more common in severe OSA compared to mild OSA (48,2 % and 10,5 %; p<0,01). The left atrial volume index (LAVI) in patients with paroxysmal AF in combination with severe OSA is greater than in patients with AF and mild and moderate OSA (58,4±11,3; 40,8±8,1 and 41,9±7,7; p<0,01). The right atrium volume index (RAVI) in patients with paroxysmal AF in combination with severe OSA is greater than in patients with AF and mild and moderate OSA (42,8±11,0; 30,2±2,2 and 32,7±8,4; p<0,05).Conclusion. Atrial fibrillation in patients with obstructive sleep apnea syndrome is more common than in the general population, and the combination of paroxysmal atrial fibrillation with severe sleep disordered breathing characterized by more pronounced atrial remodeling than when arrhythmia is combined with mild to moderate sleep apnea.