Epidemiological and clinical information on respiratory and allergic disorders in adolescents has been collected from the adolescents themselves or from their parents, but little is known about the differences between these two sources of information. This study compared the responses to 10 identically worded questions from the International Study of Asthma and Allergies in Childhood (ISAAC) questionnaire on asthma, rhinitis and eczema between written self- and parent-completed questionnaires in a large sample of 21,068 adolescent Italians aged 13-14 yrs. The effects of sex, latitude of residence, urbanization, socioeconomic status, previous diagnosis of asthma, family history of allergic diseases and adolescent's smoking habits on the prevalence estimates obtained with the two questionnaires were analysed using multiple logistic regression. Prevalence of self-reported symptoms was significantly higher in self-reported for all questions except for lifetime eczema, which was higher in parental report. Agreement between the two sources of information was poor, although it was slightly better for questions relative to clinical diagnosis of asthma, hay fever or eczema than for related symptoms, and for respiratory with respect to cutaneous disorders. When compared with self-reported, parent-reported was consistently higher in males and was more affected by socioeconomic level and by a diagnosis of asthma or family history of allergic diseases, whereas self-reported was more sensitive to the effect of the adolescent's smoking habits. It is concluded that, in adolescents, parent- and self-reported respiratory and allergic symptoms differ significantly and are differently affected by several constitutional and environmental factors. Obtaining direct information from adolescents may be essential for a correct evaluation of respiratory symptoms.