A cross-sectional survey was conducted to evaluate the possible effects of outdoor air pollution and of parental smoking on the respiratory health of children. A total of 3092 primary schoolchildren living in two polluted areas (an industrial town, Civitavecchia, and the city of Rome) and in a rural area, were chosen. A self-administered questionnaire was filled in by the parents of 2929 children (94.2%). A broad spectrum of respiratory symptoms and illnesses were taken as outcome variables. The frequency of most outcome variables was higher among children from the polluted areas than among those growing up in the non-polluted area (e.g. asthma: odds ratio (OR) = 1.4 for Civitavecchia, OR = 1.3 for Rome). Exposure to any passive smoking increased OR of having night cough (OR = 1.8), snoring (OR = 1.4), and respiratory infections during the first 2 years of life (OR = 1.3). A further increase in risk was observed in children whose mothers smoked or if both parents were smokers (asthma, OR = 1.5). When the separate and joint effects of the two exposures were studied, the patterns of OR did not suggest synergism between the two factors. The study indicates that both air pollution and passive smoking cause an increase in respiratory symptoms in children, although there would seem to be no additional effects of the two exposures together.