This study investigates whether pregnant women and their fetuses are exposed to environmental tobacco smoke (ETS) as a result of living in apartments. We measured cotinine concentrations in serum, a biomarker of exposure to ETS, in non-smoking women's umbilical cord blood collected at delivery and in maternal blood drawn shortly after delivering a baby. Concurrently, information was collected regarding the women's housing situation, whether family members or co-workers smoked, and other potential exposure factors. Newborns whose non-smoking mothers lived in an apartment during pregnancy were more than three times (OR 3.17, 95% CI 1.62-6.21) more likely to have detectable levels of cotinine in their cord blood serum than babies whose mothers lived in a detached house. There is a strong association between detectable concentrations of cotinine in cord blood serum and living in an apartment, even after adjusting for confounders, such as exposure at home or at work. A similar association was observed between the detectable levels of cotinine in maternal serum and living in an apartment (OR 1.95, 95% CI 1.03-3.71).