The main goal of the project was to estimate the effect of environmental tobacco smoke (ETS) exposure on the birthweight. In a cohort of 196 pregnant women in 20-24 week of pregnancy the serum and urine cotinine levels were determined. The cohort included randomly selected pregnant patients of a maternity units in Lodz, Poland. To assess a 24 h exposure to ETS preceding the day of examination, both serum and urine cotinine measurements were applied. A statistically significant relationship was found between serum cotinine concentration and brithweight. The newborns of nonsmoking mothers whose serum cotinine levels were characteristic for passive smoking (2-25 ng/ml) had their birthweight lower by an average of 30 g, compared to those of women who were not exposed to ETS (serum cotinine below 2 ng/ml). It was concluded that more effective public health measures should be undertaken to ensure a tobacco smoke-free environment for pregnant women. Until this goal is achieved, pregnant women should be informed about health risks from ETS exposure so that they would avoid it both at home and workplace.