Technical Abstract: Background: We have demonstrated that non-obese adolescents increase insulin sensitivity but not insulin secretion in response to a high carbohydrate (HCHO) diet , but corresponding adaptive responses in obese adolescents are unknown. Aim: To test the hypothesis that obese adolescents are insulin resistant and, thus, will not increase insulin sensitivity but rather insulin secretion to maintain euglycemia in response to a HCHO diet. Subjects: Eight obese adolescents (6 f; 2 m; 14.7±0.2 y; BMI: 32.2±0.9 kg/m2; Body fat 42.3±2.0% (DXA); Tanner IV-V) (mean±SE) were studied twice following, in random order, 7 d of a HCHO (60% CHO; 25% fat) or a high fat (HFat) (30% CHO; 55% fat) diet. Insulin sensitivity (Si*); glucose effectiveness (Sg*) and insulin secretory indices (phi 1 and phi 2) were measured using a stable labeled FSIVGTT. Data in the obese subjects were compared to those of previously studied non-obese adolescents (6 f, 5 m; 14.5±0.3 y; BMI: 20.3±1.0 kg/m2; Body fat: 18.0±1.3% (DXA); Tanner IV-V). In addition, as compared to non-obese, obese adolescents had higher fasting insulin and C-peptide concentrations. Conclusion: Irrespective of dietary CHO content, obese adolescents secreted more insulin and were less sensitive to its action than their lean counterparts. Further, and in distinct contrast to their non-obese counterpart, obese adolescents increased insulin secretion but not insulin sensitivity in response to a high dietary CHO intake.