This study measured the urinary concentrations of bisphenol A (BPA) and 4-nonylphenol (NP) in a reference population of 394 U.S. adults using isotope-dilution gas chromatography/mass spectrometry. BPA was detected in 95% of the samples, with geometric mean and median concentrations of 1.33 μg/L and 1.28 μg/L, respectively, and a 95th percentile of 5.18 μg/L. NP was detected in 51% of the samples, with median and 95th percentile concentrations of <0.1 μg/L and 1.57 μg/L, respectively. The frequent detection of BPA suggests widespread exposure in the U.S., while the lower detection frequency of NP may be due to lower exposure, different pharmacokinetic factors, or the fact that NP is a small component of commercial mixtures. This study provides the first reference range for human internal dose levels of BPA and NP in a demographically diverse population. Despite the sample population's nonrepresentativeness of the U.S. population, the results offer valuable insights into exposure levels. BPA and NP are widely used in consumer products, leading to potential human exposure. Toxicologic studies suggest that exposure to BPA and NP may cause reproductive and developmental issues. BPA is used in plastics and resins, while NP is used in surfactants. Both can enter the environment during manufacturing or leaching from products. Human exposure to BPA and NP is high due to their widespread use. The study highlights the need for further research to determine the best urinary biomarkers for assessing exposure to NP. The results indicate that BPA is more commonly detected than NP, suggesting higher exposure to BPA. However, NP's exposure may be underestimated due to its complex metabolism and the fact that nNP, the measured isomer, represents a small percentage of commercial NP mixtures. The study underscores the importance of further research to understand the health impacts of these chemicals and to develop effective monitoring methods.This study measured the urinary concentrations of bisphenol A (BPA) and 4-nonylphenol (NP) in a reference population of 394 U.S. adults using isotope-dilution gas chromatography/mass spectrometry. BPA was detected in 95% of the samples, with geometric mean and median concentrations of 1.33 μg/L and 1.28 μg/L, respectively, and a 95th percentile of 5.18 μg/L. NP was detected in 51% of the samples, with median and 95th percentile concentrations of <0.1 μg/L and 1.57 μg/L, respectively. The frequent detection of BPA suggests widespread exposure in the U.S., while the lower detection frequency of NP may be due to lower exposure, different pharmacokinetic factors, or the fact that NP is a small component of commercial mixtures. This study provides the first reference range for human internal dose levels of BPA and NP in a demographically diverse population. Despite the sample population's nonrepresentativeness of the U.S. population, the results offer valuable insights into exposure levels. BPA and NP are widely used in consumer products, leading to potential human exposure. Toxicologic studies suggest that exposure to BPA and NP may cause reproductive and developmental issues. BPA is used in plastics and resins, while NP is used in surfactants. Both can enter the environment during manufacturing or leaching from products. Human exposure to BPA and NP is high due to their widespread use. The study highlights the need for further research to determine the best urinary biomarkers for assessing exposure to NP. The results indicate that BPA is more commonly detected than NP, suggesting higher exposure to BPA. However, NP's exposure may be underestimated due to its complex metabolism and the fact that nNP, the measured isomer, represents a small percentage of commercial NP mixtures. The study underscores the importance of further research to understand the health impacts of these chemicals and to develop effective monitoring methods.