Protecting public health from environmental exposures requires four steps: detection of exposures known or expected to cause disease, assessment of health risk from exposure, implementation of an exposure intervention, and assurance that the exposure intervention is effective. To prioritize efforts in these four areas one must consider the size of the population affected, the seriousness of health effects, and the availability of cost-effective exposure interventions. Population exposure data is critical to each of these steps for protecting health. Biomonitoring data for the US population is now available to assist public health scientists and physicians in preventing disease from environmental exposures, and it complements that available for levels of chemicals in environmental media. The Second National Report on Human Exposure to Environmental Chemicals provides for the US population serum, blood and urine levels for 116 environmental chemicals over the years 1999 and 2000, with separate analyses by age, sex, and race/ethnicity. This national exposure information identifies which chemicals get into Americans in measurable quantities; determines whether exposure levels are higher among population subgroups; determines how many Americans have levels of chemicals above recognized health threshold levels (for chemicals with such threshold levels); establishes reference ranges that define general population exposure so unusual exposures can be recognized; assesses the effectiveness of public health efforts to reduce population exposure to selected chemicals; and tracks over time trends in US population exposure. Blood lead measurements in the population were important in identifying lead in gasoline as a significant source of human lead exposure and documenting the reduction in blood lead levels in the population as a result of removing lead from gasoline and other products in the United States. Serum cotinine levels in the early 1990s found more widespread exposure to environmental tobacco smoke (ETS) in the United States than previously thought and additional measurements in 1999 and 2000 documented major declines in exposure to ETS as a result of public health actions in the 1990s. A new biomonitoring assessment of the exposure of the US population will be released every 2 years as the "National Report on Human Exposure to Environmental Chemicals." These reports will include the current 116 chemicals and new chemicals added to monitor priority exposures of the population.