Objectives: To assess the prevalence of the metabolic syndrome (MetS) in overweight/obese children and adolescents of an out-patient clinic, and to compare two definitions of MetS in adolescents. Methods: In total, 528 overweight/obese children (3-16 years), of multi-ethnic origin, underwent an oral glucose tolerance test, blood collections and anthropometric measurements. In children < 10 years, MetS was assessed according to childspecific cut-off values (MetS-child). In adolescents, MetS-child and MetS-adolescent (the recommendation of the International Diabetes Federation for adolescents) were compared. Results: The prevalence of MetS-child within the cohort (median age 11.3, range 3.1-16.4 years) was 18.6% (children < 10 years vs. adolescents: 14.1% vs. 20.7%, P=0.073). Insulin resistance was present in 47.7% (children < 10 years vs. adolescents: 21.8% vs. 60.1%, P < 0.001). MetS-child was highly prevalent, and not statistically significant between age groups. In adolescents, the prevalence of MetS-adolescent was higher than MetS-child (33.2% vs. 20.7%, P < 0.001). The agreement between the MetS definitions was moderate (κ=0.51), with the agreement for the MetS-criteria for abnormal lipid levels being substantial to very good (κ=0.71 to 0.80). Conclusions: MetS-child was highly prevalent in overweight/obese children and adolescents. A higher prevalence of MetS according to adolescent- as compared to child-specific criteria was found.