Introduction - In 2004, New York City conducted a population-based environmental biomonitoring study to identify exposures to selected biomarkers and provide guidance for public health actions and public policies aimed at reducing exposures. Biomonitoring Methods - As part of this study, urine and blood samples were collected and analyzed to assess concentrations of inorganic and organic mercury and urinary metabolites of organophosphate pesticides and pyrethroids (n=1,811). Biomonitoring results - The 95th percentile levels of inorganic mercury were highest among individuals born in the Dominican Republic, which was attributed primarily to the use of illegally imported mercury-containing skin-lightening creams. Total mercury levels were three times higher in New York than in the rest of the United States, with variations within the New York population primarily due to differences in fish consumption. Pesticide exposures were 4 to 14 times higher in New York than in the rest of the United States. Public policy measures and discussion - Biomonitoring led New York City to take several actions, including a product ban, a comprehensive approach to managing mercury in fish by New York City, state, and federal authorities, along with public and healthcare professional awareness campaigns, and local initiatives to reduce pesticide use and sales. The article also presents a conceptual framework to explain how and why the results of environmental biomonitoring can influence public policy. (R.A.)