There are four definitions of the metabolic syndrome that have been recommended by the World Health Organization (WHO), the European Group for Study of Insulin Resistance (EGIR), the National Cholesterol Education Program Expert Panel (NCEP), and the American Association of Clinical Endocrinologists (AACE) separately since 1998. The prevalence of the metabolic syndrome reported from different studies has varied widely, mainly because of differences in the definitions of the syndrome and in the characteristics of the populations studied. Prospective studies on the relationship between the metabolic syndrome and cardiovascular risk are still scanty. Results from several studies including a large population-based Italian study, the Framingham Offspring Study, the Botnia Study, the Kuopio Ischemic Heart Disease Study, the National Health and Nutrition Examination Survey II Mortality Study, the San Antonio Heart Study, and the DECODE study have shown that the presence of metabolic syndrome using different definitions is associated with a significantly increased risk of total mortality and cardiovascular morbidity and mortality.