Aim — to estimate the prevalence of the obstructive sleep apnea syndrome (OSA) in patients with chronic rheumatic heart disease (RHD) and its effect on the disease. Material and methods. The study included 165 patients with acquired mitral stenosis who underwent risk assessment of OSA, snoring and daytime drowsiness. Cardiac respiratory monitoring was performed to detect OSA. In assessing the functional class of chronic heart failure (CHF), a 6-minute walk test was used. In addition, patients were assessed for quality of life according to the SF-36 questionnaire and anxiety and depression scales were used. Results. According to cardiorespiratory monitoring, patients were divided into 4 groups: with a normal value of the apnea/hypopnea index (AHI) — 14.5% (24 patients); mild degree of OSA (AHI 5—14) — 58.1% (96 patients); moderate degree of OSAS (AHI 15—29) — 16.3% (27 patients); a severe degree of OSA (AHI 30 or more) — 10.9% (18 patients). With a decrease in the distance of the test of a 6-minute walk in patients with RHD (from 395.8±22.74 to 226.21±22.23 meters) statistically significantly decreased AHI. At the same time, all patients in the non-OSA group had atrial fibrillation (AF) and statistically significantly smaller area of the mitral orifice — 1.17±0.07 cm2. Dyspnea in the VAS in patients without OSA was higher — 53.0±5.3 mm, but the data were unreliable. There is no statistically significantly difference in the quality of life questionnaire SF-36, the scales of anxiety and depression have not been received. Holter monitoring showed statistically significantly higher frequency of 2 second pauses in the background of AF and a larger number of ventricular extrasystoles in groups of patients with OSAS. Conclusion. In patients with RHD there is a high prevalence of OSA — 85.5%. The severity of AHI in patients with RHD is statistically significantly reduced by removing the distance of the test of a 6-minute walk. At the same time, OSA does not affect the quality of life, the severity of anxiety and depression in patients with RHD, which may be due to the effect of CHF on these indicators. The presence of OSA in patients with RHD affects the slowing of atrio-ventricular conduction and an increase in the number of ventricular extrasystoles.