We investigated the prevalence of metabolic syndrome in patients with obstructive sleep apnea syndrome (OSAS) referred to a tertiary university-based medical center. A cross-sectional study of patients with a definite diagnosis of OSAS was performed using new diagnostic criteria for metabolic syndrome that were designed for the Japanese population. Clinical features and comorbidities related to metabolic syndrome were compared between 819 patients with OSAS (719 men and 100 women) and 89 control subjects without OSAS. Metabolic syndrome was significantly more common in the patients with OSAS than in the controls (49.5% vs. 22.0% for men, p < 0.01; 32.0% vs. 6.7% for women, p < 0.01). Men with OSAS (apnea-hypopnea index [AHI] > or =5/h) had a higher risk of metabolic syndrome compared with controls (odds ratio [OR]: 3.47; 95% confidence interval [CI]: 1.84-6.53). There was a significantly increased risk of metabolic syndrome in men with moderate OSAS (AHI: 15-29.9/h) (OR: 2.83; 95% CI: 1.42-5.66) and men with severe OSAS (AHI > or =30/h) (OR: 5.09; 95% CI: 2.67-9.71). Women with OSAS (AHI> or =5/h) also had an increased risk of metabolic syndrome (OR: 6.59; 95% CI: 1.47-29.38), and the risk was significantly higher in women with severe OSAS (AHI > or =30/h) (OR 14.00; 95% CI: 2.93-66.82). Risk factors for metabolic syndrome differed by gender: in men, age, body mass index (BMI), and OSAS (AHI > or =15/h) were significantly associated with metabolic syndrome, whereas, in women, BMI was the only risk factor for metabolic syndrome. The increase of metabolic syndrome in Japanese OSAS patients suggests that this patient population is burdened with multiple risk factors for cardiovascular disease.