Obstructive sleep apnea (OSA) shares many cardiovascular risk factors with metabolic syndrome, including obesity, hypertension, insulin resistance, and pro-inflammatory state. This study aimed to examine the possible association of OSA severity with insulin resistance, inflammation and the metabolic syndrome. Ninety eight patients suspected for OSA (54.9+/-13.1 years) were studied. Overnight polysomnography and blood sampling was taken for glucose, insulin, high-density lipoprotein(HDL)-cholesterol, triglycerides, high-sensitivity C-reactive protein (Hs-CRP), and serum amyloid A (S-AA). Insulin resistance was estimated by the homeostatic model assessment (HOMA). Each patient was assigned a metabolic score according to the number of discrete components of metabolic syndrome identified, and categorized by OSA severity. Nine patients had primary snoring, nine had mild, 27 moderate and 53 severe OSA. Metabolic score increased from 1.56+/-1.01 to 2.92+/-1.20 with OSA severity (p=0.004), and was correlated independently with apnea hypopnea index (AHI; r=0.432, p=0.001) and with body mass index (BMI; r=0.518 p=0.001). Hs-CRP increased from 3.44+/-4.25 to 5.87+/-4.76mg/dL with OSA severity (p=0.066) and correlated with AHI (r=0.348; p=0.002). Insulin resistance, correlated significantly with AHI (r=0.390 p=0.021). Inflammation, insulin resistance and metabolic syndrome increase with OSA severity. The number of cardinal features of metabolic syndrome increases with an increase in OSA severity, regardless of the BMI.