In order to study the nutritional pattern in obese subjects living in an area with a Mediterranean diet, as well as the modifications in nutritional behaviour, cardiovascular risk factors and insulin sensitivity induced by changes in nutritional pattern, 54 obese patients completed a 20-week behaviour programme. They prospectively fulfilled a food records diary in order to ascertain their nutritional pattern. Weight, body mass index (BMI). waist-to-hip ratio (WHR), blood pressure (BP), cholesterol, HDL-c, LDL-c, triglycerides, fasting and 2 h-post glucose load plasma glucose were determined at the onset and at the end of the study. Insulin sensitivity index (SI), and glucose effectiveness (Sg) were assessed by using the modified FSIVGTT. The usual diet in obese patients living in a Mediterranean country is low in carbohydrates (35%) and high in fats (43%), 55% of the latter being monounsaturated fatty acids (MUFAs), especially olive oil. After the educational programme they decreased the caloric intake to slightly over 700 Kcal/day, with a reduction of 36% in carbohydrates consumption, 18% in proteins and 43% in fats (46% in MUFAs). These modifications resulted in a decrease in weight, BMI, WHR, BP, and fasting and 2 h-post glucose load plasma glucose (all p < 0.05), whereas the lipoprotein profile did not change. In a similar way SI and Kg (glucose disappearance rate) increased, while fasting plasma insulin (FPI) decreased (p < 0.05) and Sg and I1+3 remained unaffected. Our results indicate that weight loss induced by caloric restriction improves insulin sensitivity rather insulin secretory capacity or glucose effectiveness, and all the cardiovascular risk factors but lipoproteic profile, that remains unchanged, probably because of the lower MUFAs consumption. These facts should be taken into account when recommending changes in the diet of obese patients with a Mediterranean-style diet.