BACKGROUND: Obesity in adolescence is a serious health problem because it may proceed to adulthood, cause metabolic complications, and thereby increase mortality. The main purpose of this study was to investigate the prevalence of obesity and other metabolic complications related to obesity among Korean adolescents. METHODS: The subjects of the study were 3,615 adolescents recruited from 12 middle schools nationwide. We measured their height, weight, obesity indices, fasting blood glucose, serum total cholesterol, triglyceride, HDL- cholesterol, ALT, AST, and uric acid levels. The risk factors of obesity were assessed by self-administered questionnaires. RESULTS: Obesity was evident in 16.2% of the subjects (20.7% in boys, 11.0% in girls). Adolescent obesity was closely related to parental obesity (Odds Ratio 2.9, 95% CI (Confidence Interval) 1.7~2.5). Obese adolescents were at higher risk of elevated AST (OR 10.2, 95% CI, 5.2 ~19.9), elevated ALT (OR 12.9, 95% CI, 6.9~24.2), hypercholesterolemia (OR 1.5 95% CI, 1.2~1.8, P<0.05), hypertriglyceridemia (OR 4.0, 95% CI, 3.1~5.3), and hyperuricemia (OR 2.2, 95% CI, 1.7~3.0). More than 70% of obese adolescents had one or more metabolic complications related to obesity. CONCLUSION: Adolescent obesity was closely related to parental obesity and metabolic complications related to the obesity were common among both obese boys and girls.