Many studies have documented a strong association of active smoking during pregnancy with fetal growth retardation. Increasing interest has also been focused on whether there is an association between exposure of pregnant women to environmental tobacco smoke (ETS) and low birthweight of their babies. In the intervention controlled study "Healthy Pregnancy--Healthy Child", mothers after delivery were interviewed by medical students who collected data about their smoking and nutrition. Students were also trained to stimulate non-smoking behaviour and to explain the risks related to smoking and exposure to ETS. Data from 1147 mothers after delivery were collected but only single births were included in the analysis of birthweight. In our study, 63.4% women never smoked and 32.2% women reported they had stopped smoking either before pregnancy or during the first trimester. Only 4.4% of mothers (n = 50) smoked during the whole pregnancy. Women with the history of smoking were exposed to ETS more often than mothers who never smoked (51.6% vs 17.4%; p < 0.001). The number of heavily exposed both at home and workplaces was more than twice higher among former smokers compared with never smokers (22.4% versus 9.4%, p < 0.01). The average birthweight of babies born to women who had stopped smoking was higher than that born to never smokers. The average birthweight of babies born to women who smoked during pregnancy was lower by 119 g and 171 g than that of the babies born to never smokers and former smokers, respectively. When pre-term neonates were excluded, differences in birthweight between babies born to never smokers and either formerly smoking or still smoking mothers were slightly lower. The greatest effect of ETS exposure on birthweight was recorded in never smoking mothers; an average reduction in birthweight was 88 g. A strong dose-effect was observed; in mothers heavily exposed to ETS both at home and at work, the babies' birthweight was lower by 189 g in comparison with the group of non-exposed, never smoking mothers and even by 70 g compared with mothers smoking during pregnancy.