The extent of metabolism of [D10]phenanthrene to [D(10)]r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetradeuterophenanthrene ([D10]PheT) could be a biomarker of human metabolic activation of carcinogenic polycyclic aromatic hydrocarbons, leading to identification of smokers particularly susceptible to lung cancer. The longitudinal stability of [D10]PheT was evaluated in 24 cigaret smokers given 7-8 oral doses of [D10]phenanthrene (10 µg) over 5.5 months. [D10]PheT in 6 h urine was quantified after each dose. The overall coefficient of variation for 24 subjects was (mean ± S.D.) 27.4% ± 8.83%. Thus, a single administration of [D10]phenanthrene is likely sufficient to determine a smoker's ability to metabolize it to [D10]PheT.