OBJECTIVES:To compare blood lipids, lipoproteins, apoproteins, ®brinogen, insulin and aerobic capacity in obese and nonobese Chinese Singaporean boys. To examine relationships between blood metabolites, body composition and aerobic capacity in these groups. DESIGN: Cross-sectional SUBJECTS: Forty Chinese Singaporean boys aged 13 ± 15 y. Classi®ed as obese (n 20) or non-obese (n 20) based on adiposity (fat massafat free mass): b 0.60 obese,`0.40 non-obese. MEASUREMENTS: Body composition (dual energy X-ray absorptiometry), waist circumference, peak oxygen consumption (VO 2 peak), serum concentrations of total cholesterol, triacylglycerol, high density lipoprotein cholesterol (HDL-C), total cholesterolaHDL-C, apoproteins AI and B, lipoprotein(a), insulin and glucose. Plasma concentration of ®brinogen. RESULTS: Obese boys had signi®cantly (P`0.01) higher (mean AE s.d.) concentrations of serum triacylglycerol (1.51 AE 0.65 vs 1.04 AE 0.34 mmolal), serum insulin (24.1 AE 11.5 vs 12.3 AE 4.45 mUal) and plasma ®brinogen (4.01 AE 0.54 vs 3.35 AE 0.76 gal) than non-obese boys. Within the non-obese group plasma ®brinogen concentration was signi®cantly related to percentage body fat (r 0.546, P`0.05). VO 2 peak relative to body mass (mlakgamin or mlakg 70.67 amin) was signi®cantly (P`0.001) lower in obese compared to non-obese boys but absolute VO 2 peak (lamin), adjusted for fat-free mass via analysis of covariance, was higher in obese than non-obese boys (P`0.01). Partial correlations revealed that none of the blood metabolites were signi®cantly related to VO 2 peak independent of body fatness. CONCLUSIONS: Obesity was related to elevated concentrations of serum triacylglycerol, serum insulin and plasma ®brinogen in Chinese Singaporean boys. These elevated concentrations did not appear to be associated with a lower aerobic capacity (independent of body fatness) in the obese. International Journal of Obesity (2001) 25, 984 ± 989