The SIDRIA project (Italian Studies on Respiratory Disorders in Children and the Environment) is a large multicentre, population-based survey within the ISAAC study (International Study on Asthma and Allergies in Childhood), aimed at estimating the prevalence of respiratory and allergic disorders in Italian children and adolescents, and at investigating the role of several potential risk factors for respiratory disorders in childhood. Two cross-sectional investigations (SIDRIA phase I and II) were carried out in 1994-95 and 2002 using ISAAC methods, and the same sampling design; more than 35,000 children (6-7 years old) and adolescents (13-14 years old) were enrolled in each phase. The results of the study allow international comparisons of the prevalence of asthma and allergies in childhood as well as the comparison of time trends in the prevalence of these disorders. In this paper we present an overview of the main findings of SIDRIA, focusing on the results of the second phase of the study, and in particular on the changes in the prevalence of asthma and allergies over time.