Epidemiologic data have shown increased risks of lung cancer in nonsmokers exposed to environmental tobacco smoke (ETS). We measured biomarkers in urine samples from nonsmokers before and after a 4-h visit to a casino where smoking is allowed. The tobacco-specific lung carcinogen, NNK [4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone] is a constituent of ETS. Urinary metabolites of NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and its glucuronides (NNAL-Gluc), are excellent biomarkers of human uptake of NNK and NNAL. NNAL, as with NNK, is a potent pulmonary carcinogen. Subjects collected a spot urine sample before the casino visit and all urine samples for the 24-h period starting after the visit. We analyzed samples for creatinine, total cotinine (cotinine and cotinine-glucuronide), and total NNAL (NNAL plus NNAL-Gluc). Paired samples showed statistically significant mean increases in total cotinine (0.044 nmol/mg creatinine, P < 0.0001) and total NNAL (0.018 pmol/mg creatinine, P < 0.001). These findings demonstrate that exposure of nonsmokers to ETS in a commercial setting results in uptake of a tobacco-specific lung carcinogen.