The relation between the prevalence of doctor-diagnosed asthma in children and the presence of air-proof doors and windows (doors and windows with rubber gaskets) in their homes was investigated by a cross-sectional survey in the area of Campania Plain, South Italy. Information on the occurrence of asthma and home/family characteristics, including parental smoking habit and level of education, was obtained by a questionnaire given to school children, aged 8-14 years, and their parents. After possible confounders were controlled, the risk of developing asthma was found to be significantly higher in children living in houses equipped with air-proof doors and windows (Odds ratio = 1.30, 95% Confidence interval = 1.1-1.5). By reducing the air exchange, these fixtures are likely to produce increased levels of indoor pollutants. No interaction was found between the two variables 'air-proof doors and windows' and 'parental smoking habit'.