We examined the relationship of paternal smoking habits to birthweight in a cohort of infants enrolled at birth (n = 1219). To assess validity of parental smoking reports, cord serum levels of cotinine were measured in 175 newborns. A mean birthweight deficit of 88 g was found in newborns of nonsmoking mothers whose fathers smoked more than 20 cigarettes/day. Only 2 of 138 newborns whose mothers said they were nonsmokers had cotinine levels indicating that their mothers were active smokers. Among infants of nonsmoking mothers, detectability of cotinine in cord blood was significantly correlated with the number of cigarettes smoked daily by fathers. These data suggest that fetal growth may be adversely affected when the mother is passively exposed to tobacco smoke during pregnancy.