Objective – to study the incidence and risk factors of atrial fibrillation (AF) in patients with obstructive sleep apnea syndrome (OSA), verified according to screening respiratory monitoring during sleep, performed during hospitalization in a therapeutic hospital. Design and methods. The study performed a retrospective analysis of 291 case histories of patients hospitalized in a therapeutic clinic in 2021-2022. All patients underwent overnight respiratory monitoring as part of the initial screening examination for sleep-disordered breathing. Results. Obstructive sleep apnea syndrome was identified in 216 patients, mild severity – in 27,8%, moderate severity – in 20,3%, severe severity – in 26,1% of patients. The incidence of atrial fibrillation in patients with diagnosed OSA was 28,7% and was significantly higher in patients with severe apnea compared to patients with mild apnea (p=0,043). According to the results of the analysis of the reasons underlying the development of AF in patients with verified apnea, arterial hypertension was identified in 96,8% of patients, chronic heart failure in 72,6%, coronary heart disease in 51,6%, thyrotoxicosis in 6,5%, sick sinus syndrome in 4,8% of patients, 19,4% of patients without structural heart damage. In severe apnea, the permanent form of AF was more common (p=0,008), and in mild OSA, paroxysmal AF was more common (p=0,024). It was found that the volumes of the left and right atria in patients with AF and severe OSA are greater than in patients with mild apnea. Conclusion. AF in OSA often occurs in patients without organic heart disease. The most common risk factors for AF in patients with OSA were arterial hypertension, found in 96,8% of subjects, as well as obesity (in 74,2%). The incidence of AF in patients with severe apnea is higher than in patients with mild apnea. The permanent form of AF is detected more often, and atrial volumes are larger in patients with severe OSA compared to patients with mild sleep-disordered breathing.