This study aimed to assess the relationship between birth weight and maternal exposure to passive smoking during pregnancy, and to investigate some other determinants of birth weight. A retrospective cohort study in Baghdad, Iraq was conducted during February to August 2004 on a random sample of 300 non-smoker housewife mothers, interviewed 24 hours after delivery: 150 were not exposed to passive smoking at home and 150 were exposed. The mean birth weight of exposed newborns was significantly lower than non-exposed newborns. In exposed newborns, a significant inverse relationship was noticed between birth weight and the number of cigarettes smoked by household members (r = -0.27). Multiple regression analysis showed that after controlling for all the variables studied, birth weight had a significant inverse correlation with the maternal exposure to passive smoking and a positive correlation with adequate antenatal care.