The metabolic syndrome (MS) is defined by the clustering of a number of cardiovascular risk factors. The aims of the present study were to estimate the prevalence of MS in Madrid (Spain) by 2 definitions and to investigate its relationship with several sociodemographic factors and C-reactive protein (CRP) levels. This was a cross-sectional population study, and participants were 1344 subjects aged 31 to 70 years. Clinical evaluation included data on sociodemographic and cardiovascular background, physical examination, fasting glucose, triglycerides, and high-density lipoprotein cholesterol. The CRP levels were determined in a subgroup of 843 subjects. The diagnosis of MS was made according to the 2005 Adult Treatment Panel III (ATP III) and International Diabetes Federation (IDF) definitions. The age- and sex-adjusted prevalence of MS was 24.6% (95% confidence interval [CI], 22.3%-26.9%) using the ATP III definition and 30.9% (95% CI, 28.4%-33.3%) using the International Diabetes Federation definition. The overall agreement rate was 91.5% (kappa = 0.80; 95% CI, 0.76-0.83). Prevalence figures by both definitions were higher in men than in women and increased with age. Male sex, older age, low educational level, and physical inactivity were all determinants of ATP III-defined MS. The presence of MS or any of its components was associated with high CRP levels. In a logistic regression analysis, low educational level and waist circumference were the best predictors for high CRP level. The prevalence of MS in the Madrid region is one of the highest in Europe and confirms the strong Spanish regional variability in this syndrome frequency. Some sociodemographic and lifestyle factors, particularly educational level, are predictors for MS and high CRP levels.