The obese children were selected from approximately 10, 000 elementary school students and 4, 700 junior high school students. The incidences of obesity were 2.16%, 1.69%, 2.41% and 1.45% in male and female elementary school students and male and female junior high school students, respectively. Hypercholesterolemia was observed in approximately 30% of moderately to severely obese children (obesity index>130%). Incidence of hypertriglyceridemia in the obese children was around 10%. Plasma triglyceride levels were significantly increased in moderately (126.2±11.1mg/dl) and severely (163.3±29.6mg/dl) obese male junior high school children and severely obese female junior high school children (206.5±121.5mg/dl) than control (male; 78±4.4mg/dl, female; 78±4.0mg/dl). HDL-cholesterol measured by precipitation method using dextran sulfate and magnesium chloride, were decreased in moderately (50.7±1.5mg/dl) and severely (51.1±12.9mg/dl) obese male junior high school students and moderately obese female junior high school students (51.1±12.9mg/dl) than control (male; 61±3.0mg/dl, female; 63±2.7mg/dl). Plasma apolipoproteins (apo) A-I, B, C-II and E were measured by single radial immunodiffusion method. Significant differences were observed in plasma concentrations of apo A-I and apo B between the obese and non-obese children. Plasma apo A-I concentrations were significantly higher in moderately (164.0±3.5mg/dl) and severely (166.5±6.7mg/dl) obese male elementary school students, moderately obese male junior high school students (158.2±2.2mg/dl) and moderately obese female primary school students (149.4±12.1mg/dl) than control (130±6.2mg/dl). Hyper apo A-I lipoproteinemia observed in the obese children is in contrast to the decreased apo A-I levels in the obese adults, reported by Avogaro P. et al (Prog. Biochem. Pharmacol. 19: 141, 1983). Plasma apo B concentration showed significantly higher levels in the obese children (95.8±4.6mg/dl) than control (68±2.9mg/dl). Plasma apo C-II and apo E concentrations showed tendency to increase in the obese children. The present study clarified the characteristics of metabolic abnormalities of plasma lipids, lipoproteins and apolipoproteins in the obese Japanese children.