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/µmol/L) was significantly higher (P<.001) in smokers than in passive smokers (35.9/µmol/L) or nonsmokers (32.3 µmol/L). The maternal and umbilical mean cord thiocyanate concentrations were similar in the smoking group (95 versus 72 /µmol/L). Although the umbilical cord levels in the infants of passive smokers and nonsmokers were similar (26 versus 23 /µmol/L), both levels were significantly lower than those of smokers. Most important, there was an inverse relationship between umbiiicai cord thiocyanate concentration and birth weight (P <.001). The authors found no evidence that passive cigarette smoke exposure resulted in higher maternal or umbilical cord thiocyanate concentrations than found in nonsmokers. © 1984 by The American College of Obstetricians and Gynecologists.