Obesity among children is increasingly recognized and linked to several metabolic problems. In this study, 47 children, aged 5-14 yr, with exogenous obesity were compared to 20 normal (non-obese) children to show alterations in glucose metabolism. All the obese children had body mass index > 95th percentile and weight for age > 120%. Basal and stimulated insulin and C-peptide levels were obtained during oral glucose tolerance test (OGTT). Seven children from the obese group had impaired OGTT according to WHO criteria. Mean fasting insulin levels were 26.7 +/- 14.6 microIU/ml in obese and 10.99 +/- 4.36 microIU/ml in controls; postprandial insulin levels were 70.4 +/- 56.4 microIU/ml and 22.23 +/- 6.55 microIU/ml, respectively (p < 0.001). The euglycemic glucose clamp technique was applied to 8 normal and 22 obese children. The amount of metabolized glucose (M) during clamp test is measured to identify glucose sensitivity. Mean M values were 3.24 +/- 1.35 mg/kg/min in obese and 6.525 +/- 0.770 mg/kg/min in control children (p < 0.001). As a result of this study, it seems reasonable to consider all obese children and adults as being at risk for hyperinsulinism and insulin resistance.