To assess metabolic syndrome prevalence in adult patients with OSAS. We administered a standardized clinical questionnaire and four sleep questionnaires (Berlin, Epworth Sleepiness Scale, STOP, and STOP-Bang), and measured anthropometric variables. We also measured serum glucose and lipids, and blood pressure following an overnight fast. Metabolic syndrome was diagnosed according to National Cholesterol Education Program criteria. Patients underwent an overnight ambulatory respiratory polygraphy to confirm the diagnosis of OSAS. The predictive variables were subjected to univariate and multivariate analysis in a logistic regression model. Of 1,030 screened patients, 68% were male, 92% had comorbidities and 58% had moderate-severe OSAS. Subjects with OSAS were more obese, had higher cervical and waist circumference, blood pressure and fasting serum glucose, had lower HDL cholesterol, and an increased incidence of metabolic syndrome (55.4% vs. 44.8%, p<0.013). Age, male sex, hypertension, body mass index, cervical, waist and hip circumferences, intense snoring, witnessed apnea, nocturia, and components of metabolic syndrome were associated with the risk of OSAS and its severity. Fasting blood glucose, blood pressure, and waist circumference were associated with the risk of moderate or severe OSAS, which was not significant for the alteration of blood lipids. Patients with OSAS have a high prevalence of metabolic syndrome. OSAS was associated with an increase in the cardiovascular risk factors that comprise the metabolic syndrome.