The clinical definition of abdominal obesity is currently under review. In Western societies abdominal obesity is clinically defined by increased waist circumference or diameter, and measurements of >= 94 cm in men and >= 80 cm in women are considered pathological. Increased waist circumference is a fundamental parameter for defining metabolic syndrome and is correlated with hypertriglyceridemia, which is the earliest and most consistent metabolic alteration in metabolic syndrome. The prevalence of abdominal obesity and its association with hypertriglyceridemia and related metabolic alterations is increasing in Western societies and in developing countries. The association of abdominal obesity and hypertriglyceridemia most probably indicates the presence of insulin resistance and metabolic syndrome. Moreover, abdominal obesity classifies the described alterations independently from body weight and indicates a high risk of developing cardiovascular disease and type 2 diabetes in the next few years. Individuals with abdominal obesity and metabolic alterations, that is, with metabolic syndrome, constitute a substantial percentage of the general adult population, approximately 20-30%, and 80% of the population with type 2 diabetes. Prevention of obesity in general and of abdominal obesity in particular is fundamental to combatting the high prevalence of metabolic syndrome, type 2 diabetes and cardiovascular disease, which characterizes developed countries. Moreover, programs for the early detection of individuals with abdominal obesity and metabolic syndrome should be established and aggressive treatment should be provided to reduce the high associated morbidity and mortality.