Exposure to environmental tobacco smoke (ETS) is a major risk to human health, and the home is the greatest single source of ETS for children. The authors investigated fetal exposure to paternal smoking at home during pregnancy. Korean families were included as trios of fathers, mothers, and neonates identified in 2005-2007. Sixty-three trios were finally enrolled in this study after exclusion of those in which the mother was a smoker or was regularly exposed to ETS at places other than the home. Nicotine and cotinine concentrations in hair were measured by using liquid chromatography-tandem mass spectrometry to determine long-term exposure to ETS. The difference between neonatal nicotine concentrations in the smoker and nonsmoker groups was not statistically significant. However, in the indoor-smoker group, neonatal nicotine concentrations were significantly higher than in the outdoor and nonsmoker groups (P < 0.05). Furthermore, neonatal nicotine concentrations in the outdoor-smoker group were not different from those in the nonsmoker group. These findings indicate that paternal smoking inside the home leads to significant fetal and maternal exposure to ETS and may subsequently affect fetal health. Conversely, findings show that paternal smoking outside the home prevents the mother and her fetus from being exposed to ETS.