To estimate the prevalence of respiratory disorders in children, and to investigate the roles of potential environmental risk factors, including exposure to outdoor air pollution, a large multicenter, population based survey (SIDRIA) was conducted in Italy in 1994-1995. The study enrolled more than 40,000 children. Results allowed international comparisons of the prevalences of asthma and allergies in childhood in the framework of the ISAAC (International Study of Asthma and Allergies in Childhood) study, and supplied further evidence of the adverse respiratory effects of many environmental factors. The methods and main findings of the SIDRIA study are presented, focusing on the role of outdoor risk factors.