Polycyclic aromatic hydrocarbons (PAH) are believed to be causative agents for various types of cancers in humans. Benzo[a]pyrene (BaP) is a prototypic carcinogenic PAH, which requires metabolic activation to elicit its detrimental effects. The major end product of its diol epoxide metabolic activation pathway is r-7,t-8,9,c-10-tetrahydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene (trans, anti-BaPT). Individual differences in exposure to, and metabolic activation of, carcinogenic PAH may influence cancer risk. Measurement of PAH metabolites in human urine could provide a direct way to assess individual differences in susceptibility to PAH-related cancer. In this article, we describe a sensitive and reliable method for the quantitation of trans, anti-BaPT in human urine using gas chromatography-negative ion chemical ionization-tandem mass spectrometry (GC-NICI-MS/MS). [13C6] trans, anti-BaPT was used as the internal standard. The urine was treated with β-glucuronidase and sulfatase, and then trans, anti-BaPT was enriched by solid-phase extraction with polymeric reversed phase and phenylboronic acid cartridges. The sample was silylated and analyzed by GC-NICI-MS/MS with selected reaction monitoring (SRM) for the trimethylsilyl (TMS) derivatives of trans, anti-BaPT (m/z 446 → m/z 255) and [13C6]trans, anti-BaPT (m/z 452 → m/z 261). The mean assay recovery was 44%. The instrumental on-column detection limit was about 20 amol of trans, anti-BaPT (as BaPT-TMS). trans, anti-BaPT was readily detected in all urine samples analyzed including those of 30 smokers (0.71 ± 0.64 fmol/mg creatinine) and 30 nonsmokers (0.34 ± 0.2 fmol/mg creatinine) (P = 0.0036). The results of this study demonstrate a highly sensitive and selective method for the quantitation of trans, anti-BaPT in human urine. This is to our knowledge the first study to show that smokers have significantly higher levels of trans, anti-BaPT in their urine than do nonsmokers. This method may be useful as a direct phenotyping approach to assess individual differences in uptake and metabolic activation of carcinogenic PAH.