This study measured the urinary concentrations of bisphenol A (BPA) and 4-nonylphenol (NP) in a reference population of 394 adults from the United States using isotope-dilution gas chromatography/mass spectrometry. BPA was detected in 95% of the samples at concentrations ≥ 0.1 μg/L, with a geometric mean (GM) and median concentration of 1.33 μg/L and 1.28 μg/L, respectively. NP was detected in 51% of the samples at concentrations ≥ 0.1 μg/L, with a median concentration of < 0.1 μg/L and a 95th percentile concentration of 1.57 μg/L. The frequent detection of BPA suggests widespread exposure, while the lower frequency of NP detection could be due to lower exposure, different pharmacokinetic factors, or the fact that the measured NP isomer represents a small percentage of the NP used in commercial mixtures. The study provides the first reference range of human internal dose levels of BPA and NP in a demographically diverse population, highlighting the need for further research to establish the relevance of oxidative metabolism of NP and to identify urinary metabolites of NP commercial mixtures.This study measured the urinary concentrations of bisphenol A (BPA) and 4-nonylphenol (NP) in a reference population of 394 adults from the United States using isotope-dilution gas chromatography/mass spectrometry. BPA was detected in 95% of the samples at concentrations ≥ 0.1 μg/L, with a geometric mean (GM) and median concentration of 1.33 μg/L and 1.28 μg/L, respectively. NP was detected in 51% of the samples at concentrations ≥ 0.1 μg/L, with a median concentration of < 0.1 μg/L and a 95th percentile concentration of 1.57 μg/L. The frequent detection of BPA suggests widespread exposure, while the lower frequency of NP detection could be due to lower exposure, different pharmacokinetic factors, or the fact that the measured NP isomer represents a small percentage of the NP used in commercial mixtures. The study provides the first reference range of human internal dose levels of BPA and NP in a demographically diverse population, highlighting the need for further research to establish the relevance of oxidative metabolism of NP and to identify urinary metabolites of NP commercial mixtures.