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% CI 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.