The main purpose of our study was to assess the effects of prenatal tobacco smoke on respiratory symptoms and on doctor consultations in a birth cohort of 445 infants who had no smoking mothers and who had no postnatal exposure to environmental tobacco smoke (ETS). Before and after delivery, questionnaires and interviews with mothers were administered to solicit information on prenatal and postnatal ETS exposure. Newborns were followed-up over six months of life, and respiratory outcomes such as runny or stuffed nose, cough with or without cold, difficult (puffed) breathing, wheezing or whistling in the chest irrespective of respiratory infection were considered. In addition, medical visits related to the occurrence of respiratory symptoms were recorded for each child over a six-month study period. In the multivariate Poisson regression analysis, a set of potential confounders has been taken into account such as gender of child, season of birth, gestational age, maternal education, maternal atopy, presence of moulds in households and prenatal level of personal exposure to fine particles. The adjusted rate ratio (RR) estimated for the occurrence of episodes of running nose was significantly higher in infants exposed to prenatal ETS (1.40; 95% confidence interval [CI]: 1.11-1.68) and the corresponding RR estimates for cough, difficult breathing and wheezing were 1.49 (95% CI: 1.15-1.93), 1.96 (95% CI: 1.22-3.16) and 5.12 (95% CI: 2.86-9.16). The rate ratios of doctor consultations attributable to prenatal ETS because of cough was 1.94 (95% CI: 1.49-2.54). The risk estimate for consultations due to difficult breathing was 2.77 (95% CI: 1.76-4.36), and that for wheezing was 5.86 (95% CI: 3.56-9.64). The data strongly support the view about the impact of the "in-utero" effect of passive smoking on children's respiratory health. Higher utilization rates of doctor consultations in infants attributable to prenatal ETS exposure demand the revision of public health policy, which should be focused also on cessation of smoking practices by all household members during and after the pregnancy period. (Contains 5 tables.)