In order to compare maternal and fetal exposure to compounds of tobacco smoke at the end of pregnancy, concentrations of cotinine and thiocyanate were measured in cord serum and maternal serum collected from 24 daily smoking and four non-smoking women at the time of child birth. The median level of thiocyanate (69 mumol/l) in maternal serum was about 50% higher than the median level (47 mumol/l) in cord serum (P < 0.0001) (Wilcoxon matched-pairs signed-ranks test). On the other hand, the median level of cotinine (22 ng/ml) in maternal serum was less than half of the median level (51 ng/ml) in cord serum (P = 0.0009). The correlation between maternal and cord serum was 0.77 (P < 0.0001) (Spearman's rank correlation coefficient) for thiocyanate levels and 0.90 (P < 0.0001) for cotinine. The high correlation between measurements of cotinine in maternal serum and cord serum indicates that maternal cotinine levels can be used to describe interindividual differences in fetal exposure to tobacco smoke. The placental gradients for cotinine and thiocyanate may indicate differences in maternal and fetal exposure to products of tobacco smoke.