PURPOSE: Our study aimed to evaluated sex differences in clinical features of obese high school students. METHODS: One hundred three obese high school students (body mass index [BMI]> or =85th percentile) and 51 control students (BMI<85th percentile) were enrolled in this study. Anthropometric measurements were performed. Fasting serum glucose, insulin, aspartate aminotransferase, alanine aminotransferase, total cholesterol, triglyceride, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and high-sensitive C-reactive protein were measured. Abdominal fat thickness, degree of fatty liver, and carotid intima-media thickness were measured by ultrasound. RESULTS: In control and obese groups, waist circumference was significantly longer in boys but body fat mass was significantly higher in girls. In the control group, total cholesterol and LDL-C were higher in girls. In the obese group, however, aspartate aminotransferase, alanine aminotransferase and triglyceride were higher and HDL-C was lower in boys. Preperitoneal fat thickness was significantly higher in obese girls. In obese group, the degree of fatty liver was significantly higher in boys. Carotid intima-media thickness was not significantly different between boys and girls. CONCLUSION: Obese adolescents had distinguishable sex differences in body measurements, metabolic abnormalities, abdominal fat thickness and fatty liver. We can infer that these characteristics may extend into adult obesity.