Polycyclic aromatic hydrocarbons (PAHs), a class of carcinogenic and mutagenic pollutants formed during incomplete combustion processes, are found in automobile exhausts, forest fires, cigarette smoke and certain foods such as charbroiled meat. Human exposure to environmental PAHs can be assessed by measuring levels of their urinary metabolites, hydroxy-PAHs (OH-PAHs). We have developed a method for the measurement of 24 OH-PAHs in urine. The method consists of enzymatic hydrolysis, automated liquid-liquid extraction, derivatization and quantification by gas chromatography/isotope dilution high-resolution mass spectrometry (GC/IDHRMS). The limits of detection were in the low parts-per-trillion range. The CVs were 2.9-11%, method recoveries were 46-72% and the throughput was 40 samples per day per analyst. This method has been applied to the analysis of samples from the National Health and Nutrition Examination Survey (NHANES), performed by CDC to assess exposure of the U.S. general population to environmental pollutants.