Serum thiocyanate concentrations have been used as a marker of cigarette exposure in both smokers and nonsmokers. The authors used this measure to estimate passive exposure in low-risk healthy pregnant women at term. Three groups were compared: smokers, passive smokers, and nonsmokers. The mean thiocyanate concentration (95 mumol/L) was significantly higher (P less than .001) in smokers than in passive smokers (35.9 mumol/L) or nonsmokers (32.3 mumol/L). The maternal and umbilical mean cord thiocyanate concentrations were similar in the smoking group (95 versus 72 mumol/L). Although the umbilical cord levels in the infants of passive smokers and nonsmokers were similar (26 versus 23 mumol/L), both levels were significantly lower than those of smokers. Most important, there was an inverse relationship between umbilical cord thiocyanate concentration and birth weight (P less than .001). The authors found no evidence that passive cigarette smoke exposure resulted in higher maternal or umbilical cord thiocyanate concentrations than found in nonsmokers.