The aim of this study was to evaluate the prevalence of type 2 diabetes mellitus (DM2) and impaired glucose tolerance (IGT) in obese children and adolescents and to examine insulin resistance and insulin secretion. We studied 427 asymptomatic obese patients. DM2 and IGT were diagnosed by an oral glucose tolerance test. Insulin resistance and P-cell function were assessed by using homeostasis model assessment (HOMA), insulin/glucose index (I/GI), fasting insulin and insulin sensitivity index (ISI-composite). Thirty patients showed IGT (7%) and seven had DM2 (1.6%). The mean age was 10.7 +/- 3.5 years, the diabetic group being significantly older than the normal group (p < 0.01). The mean body mass index was 30 +/- 5.3 kg/m2 without significant differences between groups. beta-Cell function declined significantly in the patients with IGT and DM2, and insulin resistance increased significantly. Given the rather high prevalence of glucose metabolism impairment, children with obesity should undergo glucose tolerance testing for appropriate therapeutic intervention.