N-acetyl-S-(2-hydroxyethyl)-L-cysteine (2-hydroxyethyl mercapturic acid, HEMA) is a urinary metabolite of several hazardous chemicals, including vinyl chloride (VC), ethylene oxide (EO), and ethylene dibromide (EDB). Information about the levels of HEMA in the general population is useful for assessing human exposures to HEMA parent compounds, including VC, EO, and EDB. To establish reference range concentrations for HEMA, we analyzed urine samples from 412 adult participants in the Third National Health and Nutrition Examination Survey (NHANES II) by using isotope-dilution high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). HEMA was detected in 71% of the samples examined. Creatinine-corrected concentrations ranged from less than 0.68 microg/g creatinine to 58.7 microg/g creatinine; the 95th percentile concentration was 11.2 microg/g creatinine; and the geometric mean and median creatinine-corrected concentrations were both 1.6 microg/g creatinine. We observed a statistically significant difference (P=0.0001) in the creatinine-corrected geometric mean concentration values of HEMA between smokers (2.8 microg/g creatinine) and nonsmokers (1.1 microg/g creatinine). The high levels of HEMA seen among smokers likely originated from HEMA-producing chemicals known to be present in tobacco smoke.