This study aimed to compare questions and tests used in asthma epidemiology with clinical diagnosis of current asthma and to assess the extent of undiagnosed asthma in Italy. Thus, 811 attenders to the second stage of the European Community Respiratory Health Survey were classified by panels of respiratory physicians as current asthmatics or not. Among those with a clinical diagnosis of asthma (n=105), 69% reported current wheezing and 68% asthma in their lifetime (ever asthma), while asthma attacks in the previous year and/or current treatment for asthma (self-reported current asthma) were mentioned by only 37%. Thirty two per cent did not mention asthma at any time, but nevertheless presented a rate of hospitalization close to that of people with self-reported current asthma. On the other hand, almost no subjects labelled nonasthmatics by clinicians (n=706) presented self-reported current asthma (99.7%), while some reported ever asthma (5%) or current wheezing (9%). A model simulation showed that, in its usual range (0-15%), asthma prevalence is markedly overestimated by the question on wheezing and underestimated by the questions on self-reported current asthma, with respect to clinical judgement. Prevalence estimates close to those obtained by clinical judgement were achieved by combining asthma-like symptoms in the previous year with the results of lung function and allergological tests, but especially by using the single question on ever asthma. In conclusion, the present results suggest that the question on ever asthma gives prevalence estimates close to those obtained by clinical judgement and that asthma is greatly underdiagnosed in Italy.