During prenatal development, the nervous system may be more susceptible to environmental toxicants, such as secondhand smoke. The authors assessed the effects of prenatal and postnatal secondhand smoke exposure on the neurodevelopment of 6-month infants. The subjects were 414 mother and infant pairs with no medical problems, taken from the Mothers' and Children's Environmental Health study. Prenatal and postnatal exposures to secondhand smoke were determined using maternal self-reports. Examiners, unaware of exposure history, assessed the infants at 6 months of age using the Bayley Scales of Infant Development. Bayley scores were compared for secondhand smoke exposed and unexposed groups after adjusting for potential confounders. Multiple logistic regression analysis was carried out to estimate the risk of developmental delay posed by SHS exposure. The multivariate model included residential area, maternal age, pre-pregnancy body mass index, education, income, infant sex, parity, birth weight, and type of feeding. After adjusting for covariates, secondhand smoke exposure during pregnancy was found to be related to a decrease in mental developmental index score, but not to a decrease in psychomotor developmental index score. In addition, secondhand smoke exposure during pregnancy was found to increase the risk of developmental delay (mental developmental index score ≤85) at 6 months. This study suggests that the infants of non-smoking women exposed to secondhand smoke are at risk of neurodevelopmental delay.