Maternal cigarette smoking has been associated with some complications of pregnancy, including low birth weight and increased morbidity. Recently, it has been reported that maternal passive smoking also affects the fetal environment and causes fetal growth disturbance. In this study, we aimed to investigate the effects of maternal passive smoking on pregnant women and their fetuses by measuring cotinine concentrations in maternal urine and umbilical cord blood. The results were as follows: 1) Among 259 pregnant women, 17 cases (6.6%) were active smokers. The women who were not aware of passive smoking at all, were only 39 cases (15.1%). More than 80% of the pregnant women smoked either passively or actively each day. 2) Cotinine concentrations in both maternal urine and umbilical cord blood increased with the increase in passive smoking. Those in maternal serum, however, did not correlate with the increase in passive smoking. 3) The relative birth weight (R.B.W.) of the newborn infants delivered by the mothers whose cotinine concentration was more than 9.0ng/ml (This value represented the mean +1.5SD of the cotinine concentration in the urine from the mother who did not passively or actively smoke) was significantly lower than that of the mothers whose cotinine concentration was less than 9.0ng/ml. It is concluded that the measurement of the cotinine concentration in maternal urine or umbilical cord blood is very useful in estimating the effects of passive smoking on pregnant women. And passive smoking as well as active smoking also has a harmful effect on the fetal growth mechanism.