Background: Cigarette smoking during pregnancy can result in fetal exposure to carcinogens that are transferred from the mother via the placenta, but little information is available on fetal uptake of such compounds. We analyzed samples of the first urine from newborns whose mothers did or did not smoke cigarettes for the presence of metabolites of the potent tobacco-specific transplacental carcinogen 4-(methylnitrosamino)-1-(3- pyridyl)-1-butanone (NNK). Methods: The urine was collected and analyzed for two metabolites of NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and its glucuronide (NNAL-Gluc). Gas chromatography and nitrosamine-selective detection, with confirmation by mass spectrometry, were used in the analyses, which were performed without knowledge of the origin of the urine samples. Results: NNAL-Gluc was detected in 22 (71%)of 31 urine samples from newborns of mothers who smoked; NNAL was detected in four of these 31 urine samples. Neither compound was detected in the 17 urine samples from newborns of mothers who did not smoke. The arithmetic mean level of NNAL plus NNAL-Gluc in the 27 newborns of smokers for which both analytes were quantified was 0.14 (95% confidence interval [CI] = 0.083-0.200) pmol/mL. The levels of NNAL plus NNAL-Gluc in the urine from these babies were statistically significantly higher than those in the urine from newborns of nonsmoking mothers (geometric means = 0.062 [95% CI = 0.035-0.110] and 0.010 [considered as not detected; no confidence interval], respectively; two-sided P<.001). NNAL plus NNAL-Gluc levels in the 18 positive urine samples in which both analytes were quantified ranged from 0.045 to 0.400 pmol/mL, with an arithmetic mean level of 0.20 (95% CI = 0.14-0.26) pmol/mL, about 5%-10% of the levels of these compounds detected in the urine from adult smokers. Conclusions: Two metabolites of the tobacco-specific transplacental carcinogen NNK can be detected in the urine from newborns of mothers who smoked cigarettes during pregnancy.