The Centers for Disease Control and Prevention released its third set of nationally representative data on serum levels of polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and non-ortho and coplanar polychlorinated biphenyls (PCBs) in the United States collected during the 2003-2004 sampling period as part of the National Health and Nutrition Examination Survey (NHANES). We utilize the serum PCDD/F data from the three NHANES sampling periods spanning 1999-2004 to assess whether there are discernable temporal trends in the United States, either for the overall population or by age. We also compare population serum data to biomonitoring equivalents (BEs) derived for PCDDs/Fs/PCBs; BEs are estimates of the concentration of PCDD/Fs/PCBs corresponding to existing exposure guidance values estimated by various governmental agencies. The serum PCDD/F data from 1999 to 2004 provide evidence that levels in the US population are declining, mirroring international trends, although the lower levels are principally observable in the 2003-2004 time period. Examining the trend by population age, from the 1999 to 2004, PCDD/F serum levels decreased by 56% for the 12-to 19-year-old group and by 38% for the 20-to 39-year olds. A slight nonsignificant decrease was observed for 40-to 59-year olds and a slight significant increase was found for 60+ year olds. Interpretation of the data across time is complicated by certain aspects of the data unique to the various sampling time periods; thus, caution should be exercised when evaluating trend information. The population mean PCDD/F/PCB TEQ is approximately equal to the BE based on the Agency for Toxic Substances and Disease Registry's Minimum Risk Level and is approximately 2-4 times lower than the remaining BEs, placing the population mean at the border between medium and low priority levels. However, certain segments of the population have levels at the medium/high priority level (e.g., the 95th percentiles for ages 60 years and older).