We report human serum levels of selected persistent organic pollutants (POPs) categorized by age, sex, and race/ ethnicity from a statistically representative sampling of the U.S. population during 2003 and 2004. The serum levels are for several chemicals listed in the Stockholm Convention on Persistent Organic Pollutants, in the Geneva Convention on Long-Range Transboundary Air Pollution, or in both. Population data for each chemical are described by geometric means and percentiles and are categorized by age, sex, and race/ ethnicity. At the 90th and 95th percentile, the dioxin total toxic equivalency (TEQ), using the 2005 toxic equivalency factors (TEFs) for all persons 12 years of age and older was 30.9 pg/g lipid (95% confidence interval (CI): 28.2-33.9 pg/g lipid) and 37.8 pg/g lipid (95% CI: 35.3-43.4 pg/g lipid), respectively. At both the 90th and 95th percentiles total TEQ increased significantly with increasing age. The population geometric mean (GM) for the total PCB concentration (sum of 35 congeners) for all persons 12 years of age and older was 0.820 ng/g whole-weight (95% CI: 0.782-0.863 ng/g whole-weight) and 134.4 ng/g lipid (95% CI: 128.9-140.0 ng/g lipid). The population 95th percentile for the total PCB concentration for all persons 12 years of age and older was 3.53 ng/g whole-weight (95% CI: 3.23-3.92 ng/g whole-weight) and 531 ng/g lipid (95% CI: 498-570 ng/g lipid). The concentrations of aldrin, endrin, gamma-HCH, and o,p'-DDT were <limit of detection for all strata in the U.S. population. Hexachlorobenzene was detected in 99.9% of the persons aged 12 years and older (GM: 15.2 ng/g lipid, 95% CI: 14.5-15.9 ng/g lipid). beta-HCH was detected in 69.5% of the persons aged 12 years and older (75th percentile: 14.0 ng/g lipid, 95% CI: 12.1-16.5 ng/g lipid). p,p'-DDT was detected in 73.8% of the persons aged 12 years and older (90th percentile: 11.9 ng/g lipid. 95% CI: 9.9-14.9 ng/g lipid). The DDT metabolite p,p'-DDE was detected in 99.7% of persons aged 12 years and older (GM: 238.4 ng/g lipid, 95% CI: 194.6-292.2 ng/g lipid). Oxychlordane(GM: 9.4 ng/g lipid, 95% CI: 8.7-10.1 ng/g lipid);transnonachlor (GM: 14.6 ng/g lipid, 95% CI: 13.1-16.5 ng/g lipid); and heptachlor epoxide (90th percentile: 13.4 ng/g lipid, 95% CI: 11.1-15.9 ng/g lipid) were detected in 82.9, 92.6, and 60% of persons aged 12 years and older, respectively. Mirexwas detected in 40.7% of persons aged 12 years and older (90th percentile: 8.3 ng/g lipid, 95% CI: <LOD-13.0 ng/g lipid). Dieldrin was detected in 87.2% of persons aged 12 years and older (95th percentile: 18.9 ng/g lipid, 95% CI: 15.8-24.5 ng/g lipid).