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Objectives: Obstructive sleep apnoea syndrome (OSAS) is a growing health concern as it is associated with serious comorbidities. OSAS is mainly related to obesity, age, gender and a narrowed upper airway is commonly seen in patients with OSAS. This study aimed to compare spirometry parameters between obese OSAS patients and non-obese OSAS patients when patients moved from sitting to supine. Methods: This cross-sectional study was conducted at Sultan Qaboos University Hospital, Muscat, Oman, between December 2009 and December 2010. Patients with severe OSAS and who were OSAS treatment naïve were recruited. Spirometry was performed in all patients in sitting and supine positions to assess forced vital capacity (FVC), forced expiratory volume in the first second (FEV1), FEV1/FVC, forced expiratory flow (FEF) 50%, FEF 25–75%, maximum forced inspiratory flow and expiratory reserve volume. The mean difference in spirometry parameters between patients in sitting and supine positions was calculated. Results: A total of 27 OSAS patients (19 males and 8 females) were included in this study. There was a significant difference in FEV1/FVC in obese and non-obese patients when changing position (P = 0.03). In addition, there was a significant change between male and female patients’ FVC percentages (P <0.05). Male patients with OSAS had reduced FVC compared to females. There was no significant difference in the remaining spirometry parameters with patients’ change of position. Conclusion: A supine position may cause lower airway obstruction in obese patients with OSAS. The reduced FVC in males possibly contributes to the high prevalence of OSAS in men compared to women.