The main purpose of this study was to identify and evaluate determinants of hair nicotine concentrations in nonsmoking women and children exposed to secondhand tobacco smoke at home. Hair samples were collected from nonsmoking women (n = 852) and from children (n = 1,017) <11 years of age living in households (n = 1,095) with smokers from 31 countries from July 2005 to October 2006. Participants' ages, activity patterns and socioeconomic characteristics including education and employment status, and hair treatment information were collected. Multilevel linear regression modeling was used to identify the main determinants of hair nicotine concentration measured by gas chromatography coupled with mass spectrometry. Increased indoor air nicotine concentration at home were associated with increased hair nicotine concentrations in nonsmoking women and children. This association was not changed after controlling for other explanatory variables. After controlling for age, length of exposure, and socioeconomic characteristics, hair nicotine concentrations in nonsmoking children and women were estimated to be increased by 3% and 1%, respectively, for a 1 μg/m³ increase in air nicotine concentration. The association between children's exposure to secondhand tobacco smoke at home and hair nicotine concentration was stronger among younger children and children with longer exposure at home. (Cancer Epidemiol Biomarkers Prev 2009;18(12):3407–14)