THREE DEFINITIONS: The metabolic or X syndrome is defined by an association of metabolic anomalies leading to an increased risk of cardiovascular complications. Today, there are at least 3 definitions of X syndrome: those of WHO, EGIR and NCEP. To varying degrees they associate increased abdominal fat, hypertension, glucose tolerance abnormality (ranging from hyperinsulinism to diabetes), and hypertriglyceridemia with low HDL cholesterol. FROM AN EPIDEMIOLOGICAL POINT OF VIEW: The prevalence of metabolic syndrome depends on the definition used and varies with the country or ethnic group considered. About 25% of the US and 10% of the French adult populations are concerned. THE RISK OF COMPLICATIONS: According to clinical trials, people with metabolic syndrome have a 2 to 4-fold increase in risk for coronary heart disease. Some of them have a particularly high risk (association of most features of the syndrome, association of an increased waist circumference and hypertriglyceridemia, presence of biological markers such as elevated C-reactive protein or microalbuminuria). Metabolic syndrome is also associated with a 4-fold increase in risk for developing diabetes.