Background: Obstructive sleep apnoea syndrome (OSAS) is widely accepted as a cardiovascular risk factor. Lately it has been considered in turn as both a component and one of the causes of the metabolic syndrome (MS). Methods: We studied 281 heavy snorers of both sexes consecutively attending a metabolic clinic. Aim was to evaluate the association of OSAS and MS in a large series of patients within an internal medicine setting. Patients underwent a clinical and biochemical work up and performed unattended polysomnography. Results: Of 226 non-diabetic snorers, 48 had primary snoring; 54 mild, 51 moderate, and 73 severe OSAS. A positive association was found between OSAS severity, central obesity indices and the mean metabolic score (p = 0.016). Prevalence of hypertension increased with OSA severity (p = 0.010). Polysomnographic indices were correlated with the metabolic score, insulin levels and central obesity indices. At regression analysis, male sex (t = 3.92; p = 0.000) and waist circumference (t = 3.93; p = 0.000) were independently associated with AHI (apnoea/hypopnoea index), while ODI (oxygen desaturation index) and waist circumference were the independent predictors (t = 2.16; p = 0.033 and t = 3.74; p = 0.000 respectively) of the metabolic score. Prevalence of OSA was 83% in 55 patients with diabetes and 34% had severe OSA. Almost all diabetics with OSA had MS. The metabolic score was higher in diabetic OSA as compared to non-diabetic OSAS (p = 0.000). Conclusions: Our findings show a high prevalence of OSAS among patients referred to a metabolic outpatient clinic because of suspected metabolic disorders and heavy snoring and suggest a strong bidirectional association between OSAS and MS. (C) 2010 European Federation of Internal Medicine. Published by Elsevier B.V. All rights reserved.