The Centers for Disease Control & Prevention has added new biomonitoring data to its “Fourth National Report on Human Exposure to Environmental Chemicals.” The information includes updated levels for 119 chemicals, as well as concentrations of 34 substances analyzed for the first time in a representative sample of the U.S. population. The updated data show that levels of the estrogenic plastics chemical bisphenol A and many perfluorinated compounds have decreased in Americans since 1999. Levels of cotinine, a metabolite of nicotine, have declined in nonsmokers, and levels of the gasoline additive methyl tert-butyl ether have also fallen since 1999. On the other hand, levels of perchlorate, a compound used in rocket fuel, have generally increased in the U.S. population. The trends for other chemicals—including endocrine-disrupting phthalates and metabolites of organophosphorus pesticides—are more complex, with the concentrations of some substances increasing and some decreasing. The updated data are based on blood and urine samples collected from more than 2,000 people who participated in CDC’s National Health & Nutrition Examination Survey in one or more of the periods 2005–06, 2007–08, and 2009–10.