OBJECTIVE- To assess the incidence and trend of the metabolic syndrome phenotype in adolescents from the Tehran Lipid and Glucose Study during 3.6 years of follow-up. RESEARCH DESIGN AND METHODS- A total of 932 adolescents, aged 10-19 years, who had complete data and returned for reassessment 3.6 years later were investigated. RESULTS- Prevalence of metabolic syndrome at baseline and after 3.6 years was 7.4 and 6.7%, respectively, based on the Adult Treatment Panel (ATP) III definitions; 3.5 and 8.0%, respectively, based on the International Diabetes Federation (IDF) definitions; 4.1 and 9.4%, respectively, based on the American Heart Association (AHA) definitions; and 13.6 and 13.4%, respectively, based on the National Health and Nutrition Examination Survey (NHANES) definitions. Incidence rates were 5.2% (95% Cl 3-6) based on ATP III, 6.8% (5-8) based on IDF, 8.3% (6-10) based on AHA, and 8.8% (6-10) based on NHANES definitions. CONCLUSIONS- Incidence of metabolic syndrome is high in Tehranian adolescents.