Copyright ©ERS Journals Ltd 1998 European Respiratory Journal

aa Asthma represents a serious public health problem affect-ing over 100 million people worldwide [1]. In recent dec-ades, the frequency and severity of asthma seems to have been increasing in several countries, both in children [2–4] and adults [5–7], even if the true extent of the problem cannot be fully assessed, given the extreme variability in epidemiological tools used for case identification [8–11] and the absence of a standard definition of asthma [12]. Even the most recent definition of asthma [1] cannot be fully applied to epidemiological research, due to the ab-sence of an objective and noninvasive measurement of air-flow limitation. Thus, in many epidemiological studies asthma has been assessed by short questionnaires based on symptoms alone, which may underestimate or overestimate the true prevalence of the disease. As an example, in the European Community Respiratory Health Survey (ECRHS) Stage I, aimed at estimating the worldwide prevalence of asthma, a mailed questionnaire with seven questions (screen-ing questionnaire), was used. An indication of the preva-lence of the disease was obtained through the prevalence of "self-reported current asthma", i.e. the self-reporting of asthma attacks in the last 12 months and/or current use of asthma drugs. The estimated prevalence ranged from 2