Obesity in adulthood is combined with vascular endothelial cell and platelet activation. In this study we evaluated whether or not such activation is already present in obese children. Forty obese (10.3 +/- 2.5 yr) and 40 nonobese (10.3 +/- 2.3 yr) children were studied. Circulating levels of soluble (s) intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), and E-selectin, as indices of vascular endothelial cell activation, were assessed in both groups. Plasma concentrations of sP-selectin and sCD40 ligand, as indices of platelet activation, were also measured. Circulating levels of highly sensitive C-reactive protein (hs-CRP) and the lipid peroxidation product 8-iso-prostaglandin (PG)F(2alpha) were evaluated because of their ability to promote vascular endothelial cell and platelet activation. Circulating levels of all of the assessed markers were higher in obese than in nonobese children (sICAM-1, +38.8 +/- 13.3%; sVCAM-1, +26.5 +/- 13.7%; sE-selectin, +31.3 +/- 17.3%; sP-selectin, +31.7 +/- 16.9%; sCD40 ligand, +36.9 +/- 22.1%; total 8-iso-PGF(2alpha), +24.0 +/- 20.2%; hs-CRP, +76.6 +/- 12.9%; P < 0.0001). Significant correlations (P < 0.004) between plasma total 8-iso-PGF(2alpha) levels and circulating sICAM-1 (r = 0.485), sVCAM-1 (r = 0.506), sP-selectin (r = 0.449), sCD40 ligand (r = 0.498), and hs-CRP (r = 0.520) concentrations were found in obese children. In conclusion, an early activation of vascular endothelial cells and platelets was present in obese children. Increased lipid peroxidation was also present in these children and likely contributed to the observed proinflammatory phenotype.