SUMMARY. Background: Obstructive sleep apnoea syndrome (OSAS) and metabolic syndrome share common pathogenetic mechanisms such as central obesity, insulin resistance, dyslipidaemia, inflammation, and cardiovascular disease. Objectives: The aim of this study was to investigate the prevalence of metabolic syndrome in patients with OSAS referred to the sleep laboratory of the Department of Pneumonology of the Medical School of the Democritus University of Thrace. Population and Method: Seventy-nine (79) subjects were studied, with medical history, clinical examination, laboratory biochemical tests and full polysomnography. Results: Twenty-one subjects (27%) did not suffer from OSAS and served as controls, while 58 subjects (73%) suffered from mild (n=17), moderate (n=8) or severe (n= 33) OSAS. In the control group 15/21 subjects (71.43%) met the diagnostic criteria for metabolic syndrome. In the OSAS group, 16/17 patients (94.18%) with mild disease, 7/8 patients (87.5%) with moderate disease and 25/33 patients (75.75%) with severe disease suffered from metabolic syndrome. No statistically significant difference in the prevalence of metabolic syndrome was detected between the groups. Conclusions: The prevalence of metabolic syndrome, although high in patients with OSAS, did not differ significantly from that in subjects without OSAS. Pneumon 2007; 20(3):240-244.