There is growing evidence that environmental tobacco smoke (ETS) exposure may negatively affect birth outcomes, especially birth weight. This study evaluates the effect of residential ETS exposure on the risk of having a low birth weight (LBW) infant and investigates whether there is a dose-response relationship. This retrospective cohort study comprised 2,206 women who participated in the 2004-2005 North Carolina Phase V Pregnancy Risk Assessment Monitoring System. Women self-reported information on ETS exposure and birth weight was obtained from birth certificates. Logistic regression was used to obtain odds ratios and 95% confidence intervals. When adjusted for marital status and income, women exposed to ETS during pregnancy had increased odds of delivering a LBW baby (OR = 1.29, 95% CI: 1.06, 1.57). After adjustment for the same covariates, a weak dose-response relationship between ETS and LBW was found (OR = 1.28, 95% CI: 1.03, 1.60 for women who were exposed to ETS generated by one cigarette smoker; OR = 1.31, 95% CI: 0.96, 1.31 for those who were exposed to ETS generated by two or more cigarette smokers). This study provides evidence of the adverse effect of residential ETS on pregnancy outcomes. The observed relationship emphasizes the health hazard that ETS exposure in the home poses to pregnant women and their unborn babies. Educational anti-tobacco campaigns and quit smoking initiatives should target both mothers and fathers to ensure smoke-free living conditions and a healthy environment for all family members.