'Obesity Disease for Japanese Children' was defined in 2002, and very recently 'Metabolic Syndrome (MS) for Japanese Children' was also defined. We therefore aimed to determine the prevalence of these two among the obese pediatric outpatients at our university hospital. The subjects were 97 children, 58 boys and 39 girls, ranging in age from 5 to 15 years. A child was considered to be obese when the body weight exceeded 120% of the standard body weight. All the subjects exceeded 120% overweight, and 58 children (35 boys and 23 girls) were over 150% overweight. Eighty five children (53 boys and 32 girls) were diagnosed with obesity disease (87.6%). Sixteen children (12 boys and 4 girls) were diagnosed with metabolic syndrome, which was 16.5% of all the subjects and 18.8% of the children with obesity disease. Fourteen of the 16 children with MS were over 10 years old. Obesity disease is diagnosed when the child has an obesity disease score of more than 6. The obesity disease score was significantly correlated with the waist circumference and the visceral adipose tissue area measured by computed tomography. The mean score of the children with MS was significantly higher than that of the non-MS group (30.2 vs. 12.3 points). In this study, it was clear that about 90% of our clinic patients are in the obesity disease group, and need therapeutic interventions. The prevalence of MS in the pediatric age is very low compared with that of adults, but MS is a high-risk category of obesity disease.