Background. Metabolic syndrome (MS) and sleep-disordered breathing (SDB) are highly prevalent in patients with diabetes mellitus type 2 (DM2). The present study examined whether there is an independent association between SDB and MS in a sample of outpatients with DM2. Methods. MS was determined in 679 patients of the DIACORE-SDB substudy, a study of outpatients with DM2. According to the National Cholesterol Education Program (NCEP) criteria, MS is defined by at least three of the following five criteria: waist circumference of >102cm (men)/>88cm (women), blood pressure of 130/85mmHg, a fasting triglyceride level of >150mg/dl, high-density lipoprotein (HDL) of <40mg/dl (men)/<50mg/dl (women), and a fasting glucose level of 110mg/dl. The apnea-hypopnea index (AHI) was assessed with a 2-channel ambulatory monitoring device and used to define the severity of SDB (AHI<15.0: no/mild SDB; AHI 15.0-29.9: moderate SDB; AHI30.0: severe SDB). Results. 228 (34%) of the 679 participants (mean age 66 years, mean body mass index (BMI) 31.2kg/m(2), and mean AHI 14/hour) had SDB. MS was significantly more frequent in patients with more severe SDB (no/mild SDB vs. moderate SDB vs. severe SDB: 72% vs. 79% vs. 85%, respectively, p=0.038). Logistic regression analysis adjusted for sex, age, obesity (BMI30 kg/m(2)), and the HOMA index showed a significant association between the AHI and the presence of MS (OR =1.039 (1.011; 1.068); p=0.007). Further, male sex, obesity, and the HOMA index were significantly associated with MS. Conclusion. SDB is significantly and independently associated with MS in outpatients with DM2.