There is concern about the effects of passive smoking during pregnancy on fetal growth. The authors conducted a case-control study of the association of maternal exposure to passive smoking during pregnancy and the risk of term small-for-gestational-age (SGA) infants in a population of white women who did not smoke during pregnancy and had only a small percentage of users of illegal drugs and alcohol. A total of 111 cases, defined as singleton term (> or = 37 weeks) infants with birth weights < or = 10th percentile for gestational age were compared with 124 term, non-SGA controls. All were identified from Contra Costa, California birth certificates for January 1-September 30, 1991. Subjects were interviewed face-to-face to collect information on exposure to passive smoking during pregnancy. The estimated relative risk for term SGA in association with passive smoke exposure during pregnancy was not increased (> or = 30 hours: odds ratio (OR) = 0.41, 95% confidence interval (Cl) 0.12-1.29) and the risk of term SGA did not increase with increasing hours of exposure to passive smoking. Controlling for parity, weight gain, prepregnancy weight, maternal age, prenatal care, education, income, alcohol consumption, and work during pregnancy in multivariate analysis did not change the findings (> or = 30 hours: OR = 0.47, 95% Cl 0.13-1.69). In this study, maternal exposure to passive smoking during pregnancy was not associated with an increased risk of term SGA. Although difficulties in precisely assessing exposure limit these findings, they should provide some reassurance to women who cannot avoid passive exposure to cigarette smoking during pregnancy.