BACKGROUND: Infantile obesity is associated with metabolic disturbances (hyperinsulinism, impaired glucose, dyslipidemia) that determine a higher risk of type 2 diabetes, high blood pressure and atherosclerotic vascular disease in adulthood. Insulin resistance is a central mechanism of complications of obesity and is associated to body fat mass. AIM: To investigate insulin sensitivity and its association with anthropometric and metabolic variables in obese children. PATIENTS AND METHODS: We studied 314 children aged 6 to 15 years. Of these, 235 had a body mass index (BMI) over the 95 percentile for age and sex (classified as obese) and 79 had a normal body mass index; 161 were pre-pubescent and 153 were pubescent. Weight, height, percentage of total body fat (% TBF) using the sum of 4 skinfolds, blood pressure (BP), waist circumference (WC), basal glucose (Glu) and insulin (Ins) were measured. Insulin sensitivity (IS) was calculated with the homeostasis model assessment (HOMA) and QUICKI index. Serum lipids, postprandial Glu and Ins were measured only in obese children. RESULTS: BMI, WC and % TBF had an inverse and significant correlation with basal IS (p <0.001). Obese children had higher BP, basal Ins, and HOMA and a lower QUICKI index, compared to normal weight children. A low basal IS was present in 58% of obese children. Obese children with low IS had higher WC, % TBF, triglycerides, total/HDL-cholesterol ratio, basal and 2-h post glucose Gli and Ins and lower HDL-cholesterol than obese children with normal IS. In prepubescent children, the risk of a low IS was 2.43 times higher with a TBF over 33%. In pubescent children, it was 2.92 times higher with a TBF over 37%. CONCLUSIONS: Low IS in prepubescent and pubescent obese children is associated with central obesity and a higher cardiovascular.