Aim: The aim of this study was to investigate the prevalence of metabolic syndrome according to different metabolic syndrome definitions in children and adolescents with congenital adrenal hyperplasia due to 21-hydroxylase deficiency. Material and Methods: A total number of 45 patients (31 patients with classical congenital adrenal hyperplasia due to 21-hydroxylase deficiency and 14 patients with non-classical congenital adrenal hyperplasia) were enrolled to the study. The anthropometric, clinical, hormonal findings and the dose of hydrocortisone were analyzed starting from the initial day of diagnosis until the beginning of our study and the metabolic controls (good-bad) were evaluated in the follow-up period. At the last visit, systemic and anthropometric examinations (involving measures of height, weight, waist circumference and blood pressure) was performed by the same physician. Serum lipid levels were examined and oral glucose tolerance tests were performed. Metabolic syndrome prevalence in our patients was calculated with respect to the modified criteria of WHO, IDF and NCEP ATP III. Results: Metabolic syndrome was diagnosed in only 1 (2.2%) of the 45 patients in the study according to modified WHO definition, 8 patients (17.8%) received a diagnosis of metabolic syndrome according to NCEP ATP III definition. Metabolic syndrome was diagnosed in 1 (3.3%) of 30 patients above 10 years of age according to IDF definition and 20% of patients aged 6-10 years were considered to be risky for development of metabolic syndrome. Conclusion: It was found that the prevalence of metabolic syndrome was highest when NCEP ATP III definition was used.