The treatment of the metabolic syndrome aims to improve insulin sensitivity and correct/prevent the associated metabolic and cardiovascular abnormalities. Since many individuals with the metabolic syndrome are overweight, dietary treatment should be primarily focused on weight reduction. This approach can improve insulin sensitivity and exert beneficial effects on all the other abnormalities clustering in the syndrome. Insulin sensitivity can also be influenced by diet composition. In this respect, the specific effects of the quality of dietary fat are of great interest, given the considerable evidence in experimental animals that saturated fat in the diet may lead to insulin resistance. In man, there is indirect evidence that a higher saturated fat intake is associated with impaired insulin action. Human studies have also attempted to evaluate the relationship between total fat intake and insulin sensitivity. They are consistent in showing that fat intake is correlated with both plasma insulin values (positively) and insulin sensitivity (negatively). However, these correlations are largely mediated by body weight. Conversely, intervention studies are consistent in showing that when total fat intake is moderately increased (from 20 to 40%), no major effect is observed on insulin sensitivity. We have recently undertaken a large, multicentre intervention study in 162 healthy individuals given either a high-saturated-fat or a high-monounsaturated-fat diet for 3 months. It shows that a high-monounsaturated-fat diet significantly improves insulin sensitivity compared to a high-saturated-fat diet. However, this beneficial effect of monounsaturated fat disappears when total fat intake exceeds 38% of total energy. Independently of its effects on insulin sensitivity, diet composition can influence the factors clustering in the metabolic syndrome. Dietary carbohydrate increases blood glucose levels, particularly in the postprandial period, and consequently also insulin levels and plasma triglycerides. The detrimental effects of a high-carbohydrate diet on plasma glucose/insulin, triglyceride/HDL or fibrinolysis occur only when carbohydrate foods with a high glycaemic index are consumed, while they are abolished if the diet is based largely on fibre-rich, low-glycaemic-index foods. In conclusion, weight reduction is a powerful measure for the treatment of metabolic syndrome. Moreover, the diet for the treatment of the metabolic syndrome should be limited in the intake of saturated fat, while high fibre/low-glycaemic-index foods should be used without specific limitations. Moderate amounts of monounsaturated fat could be permitted as they do not induce detrimental metabolic effects.