The aim of this study was to evaluate the prevalence of the metabolic syndrome, and to examine an association between the metabolic syndrome and obesity in Korean adolescents. The data for analysis were obtained from the Korean National Health and Nutrition Examination Survey 1998 and 2001. The subjects aged 12-19 years were 1317 in 1998 and 848 in 2001. A diagnosis of the metabolic syndrome is made when three or more of the following risk factors are present: (1) waist circumference >or= 90th percentile, (2) systolic or diastolic blood pressure >or= 90th percentile (3) triglyceride >or= 1.24 mmol/L, (4) high-density lipoprotein cholesterol <or= 1.03 mmol/L, and (5) fasting glucose >or= 6.1 mmol/L. The overall prevalence of the metabolic syndrome increased significantly from 6.8% in 1998 to 9.2% in 2001 (P=0.037). The prevalence of the metabolic syndrome increased significantly with severity of obesity (P for trend <0.05). Approximately 50% of obese boys and 40% of obese girl had the metabolic syndrome. The prevalence of the metabolic syndrome has increased rapidly, and approximately 500,000 out of 5.4 million Korean adolescents aged 12-19 years might have the metabolic syndrome. These findings indicate the importance of prevention and management of the metabolic syndrome in adolescents.