This paper illustrates the principles of construction and validation of an epidemiological questionnaire by using various aspects of the questionnaire prepared for the Epidemiological Study of the Genetic and Environmental Factors in Asthma, Bronchial Hyper-responsiveness and Atopy (EGEA). Standardised international questionnaires (for adults and children) were adapted and augmented for the requirements of the study. New areas in relation to international epidemiological studies are described (detailed descriptions of asthma and allergic rhinitis, trigger factors exposure tovarious environmental factors and family history). Various aspects of validation are discussed: the acceptibility by the study of missing data in the description of asthmatic symptoms, the construct validity for a score for allergic rhinitis, the reliability of a new self-administered questionnaire for perceived hyper responsiveness to various stimuli and the validity of reported family history using information obtained from family members. Some of these elements could be used in the context of other clinical and epidemiological studies. The complete questionnaire, together with the source of the questions, instructions for interviewers and the method of coding are presented in an appendix available on the internet (http://www.splf.org/bbo/revues-articles/RMR/depotElectronique/2001-110_Kauffmann/Kauffmann2002.htm) which supplements the printed paper.