Objective. First, to study tobacco exposure among pregnant Finnish women in 1986 and 1990. Second, to study the objectively measured exposure in different socioeconomic classes and in different geographical regions. Third, to assess the value of serum thiocyanate concentrations in estimating fetal tobacco exposure.Design. Tobacco exposure was measured from maternal and umbilical serum samples by measuring the concentration of nicotine metabolite, cotinine. In addition, the concentrations of thiocyanate were measured from umbilical serum samples. The occupations of the mothers were obtained from the Finnish Birth Registry, and the area of residence from laboratory records.Setting. Finland.Subjects. 1323 infants born in late February‐early March in 1991, 1263 mothers of these infants during antenatal visits in Autumn 1990, and 976 pregnant mothers during antenatal visits in Autumn 1986. The mothers and infants studied represented all newborn infants and their mothers during one week in one country.Measurements and main results. In 1986, 21.3% of Finnish mothers and in 1990 21.2% were exposed to tobacco. In 1986, exposed mothers were on average 1.4 (95% confidence interval 0.9‐1.8) years younger than nonexposed mothers, and in 1990, exposed mothers were on average 1.6 years (95% confidence interval 1.1‐2.3) younger than nonexposed mothers. 28.5% of mothers classified as unskilled workers were exposed, but only 9.1% of those classified as upper white‐collar. There were no significant geographical differences in exposure. Although umbilical serum thiocyanate levels were always elevated in exposed infants, high serum thiocyanate concentrations were detected in infants not exposed to tobacco.Conclusions. More than 20% of pregnant Finnish mothers and their fetuses were exposed to tobacco in 1986 and 1990. Young women and women in low socioeconomic classes were the most seriously exposed. Measurement of serum thiocyanate concentrations did not yield reliable estimates of e