Background - Different definitions of the metabolic syndrome have been proposed. Their value in a clinical setting to assess cardiovascular disease (CVD) risk is still unclear. We compared the definitions proposed by the National Cholesterol Education Program Adult Treatment Panel III (NCEP), World Health Organization (WHO), European Group for the Study of Insulin Resistance (EGIR), and American College of Endocrinology (ACE) with respect to the prevalence of the metabolic syndrome and the association with 10-year risk of fatal and nonfatal CVD. Methods and Results - The Hoorn Study is a population-based cohort study. The present study population comprised 615 men and 749 women aged 50 to 75 years and without diabetes or a history of CVD at baseline in 1989 to 1990. The prevalence of the metabolic syndrome at baseline ranged from 17% to 32%. The NCEP definition was associated with about a 2-fold increase in age-adjusted risk of fatal CVD in men and nonfatal CVD in women. For the WHO, EGIR, and ACE definitions, these hazard ratios were slightly lower. Risk increased with the number of risk factors. Elevated insulin levels were more prevalent in subjects with multiple risk factors, but metabolic syndrome definitions including elevated insulin level were not more strongly associated with risk. Conclusions - The metabolic syndrome, however defined, is associated with an approximate 2-fold increased risk of incident cardiovascular morbidity and mortality in a European population. In clinical practice, a more informative assessment can be obtained by taking into account the number of individual risk factors. © 2005 American Heart Association, Inc.