Title: Relationship between body mass index and severity of obstructive sleep apnoea Objectives: The primary objective of this study was to determine whether there was an association between body mass index (BMI) and the severity of obstructive sleep apnoea (OSA). Other objectives were to investigate the effect of BMI on increased daytime sleepiness in patients, to investigate the relationship between BMI and the severity of oxygen desaturation, and finally to investigate whether BMI is related to snoring frequency. Study Design: This is a cross-sectional study with historic data Patients and Methods: In this research 70 patients were involved, all who were diagnosed with OSA using polysomnography (PSG) and were determined the severity of OSA, based on the AHI index. The study used demographic data of patients (such as age, sex, height, weight, BMI, employment), data of alcohol consumption, cigarette smoking, hypertension and diabetes, the result of the Epworth scale, the result of STOP-BANG questionnaire and polysomnographic parameters (such as the number and duration of apnoeas and hypopnoeas, AHI, saturation with the lowest desaturation, and the number of episodes and duration of snoring). Results: The study involved 53 men and 17 women. Polysomnography analysis confirmed 49 subjects with severe OSA, of whom 40 were obese (82%). 12 out of 13 third-degree obese people suffer from severe OSA. The growth of the results of the STOP-BANG questionnaire correlates with the degree of OSA. Saturation was significantly reduced in obese subjects, and snoring and the percentage of snoring in sleep were significantly increased. Conclusion: Based on the results of the study, the association of obesity with the degree of OSA and the increase in AHI was confirmed, followed by the association of obesity with a decrease in mean saturation and lowest desaturation and a significant increase in snoring episodes, snoring duration and its percentage in sleep with BMI. Significant effects of obesity on daytime sleepiness have not been reported. Keywords: