The aim of the present study was to determine in adolescents the relationship between insulin levels and body mass index (BMI), body fat distribution, diet, life style and lipid profile. We studied 167 adolescents (68 boys and 99 girls) whose ages ranged from 14 to 17 years. A detailed medical (including pubertal stage) and nutritional record was obtained from each subject. Biochemical measurements included fasting serum insulin, glucose, total cholesterol (TC), triglycerides (Tg), HDL-C, LDL-C and VLDL-C. HOMA insulin resistance (IR) and HOMA beta-cell function (beta-cell) were calculated. Insulin levels were over 84 pmol/L (cut off normal value in our lab) in 56% of the boys and 43% of the girls. Thirty-seven percent of lean adolescents whose BMI was 21.5 +/- 1.9 kg/m2 presented higher fasting insulin levels. HOMA IR, Tg, systolic (SBP) and diastolic blood pressure (DBP) values when compared to a lean normoinsulinemic group. Insulin levels were correlated (p < 0.01) with body mass index. Both boys and girls in the highest BMI quartile (BMI > 24 kg/m2) had significantly higher serum insulin, HOMA beta-cell, and Tg levels, and the lowest HDL-C levels. A high-energy intake rich in saturated fat and low physical activity were found in this lean but metabolically altered adolescents. We conclude that even with a BMI as low as 21 kg/m2 an inappropriate diet and low physical activity might be responsible for the high insulin levels and dislipidemias in adolescents.