Objective: To investigate whether children and adolescents exhibiting an impaired glucose metabolism are more obese at treatment entry and less likely to reduce their degree of obesity during treatment. Methods: The present study is a longitudinal observational study, including children and adolescents from the Children's Obesity Clinic, Holbæk, Denmark. Anthropometrics, pubertal development, socioeconomic status (SES), and fasting concentrations of plasma glucose, serum insulin, serum C-peptide, and whole blood glycosylated hemoglobin (HbA1c) were collected at treatment entry and at follow-up. Proxies of Homeostasis Model Assessment 2-insulin sensitivity (HOMA2-IS) and Homeostasis Model Assessment 2-β-cell function (HOMA2-B) were calculated with the Homeostasis Model Assessment 2 program. Results: In total, 569 (333 boys) patients, median 11.5 years of age (range 6-22 years), and median body mass index (BMI) z-score 2.94 (range 1.34-5.54) were included. The mean BMI z-score reduction was 0.31 (±0.46) after 13 months (range 6-18) of treatment. At treatment entry, patients with impaired estimates of glucose metabolism were more obese than normoglycemic patients. Baseline concentration of C-peptide was associated with a lower weight loss during treatment in girls (P =.02). Reduction in the insulin concentrations was associated with reduction in BMI z-score in both sexes (P <.0001, P =.0005). During treatment, values of glucose, HbA1c, HOMA2-IS, and HOMA2-B did not change or impact the treatment outcome, regardless of age, sex, SES, or degree of obesity at treatment entry. Conclusion: The capability to reduce weight during multidisciplinary treatment in children and adolescents with overweight/obesity is not influenced by an impaired glucose metabolism at study entry or during the course of treatment.