In the early 1990s a multicentre survey on asthma was performed on the young adult population (European Community Respiratory Health Survey - ECRHS). This study is to be repeated in order to estimate changes in the prevalence of asthma-like symptoms during the last decade and to assess the social and economic costs of the disease and their variations among countries. The self-administered questionnaire devised for this purpose is a two-page questionnaire. The first page contains the same items as those used in the first survey with four additional questions related to: 1) the frequency and severity of asthma attacks; 2) the presence of chronic bronchitis; 3) smoking habits; and 4) a visual analogue scale assessing perception of outdoor pollution. The second page aims to collect information regarding the direct and indirect costs of asthma. The influence of the length of the questionnaire on the response rate was assessed in a pilot study in Italy. Two random samples of 150 subjects received either the one-page questionnaire (first page) or the two-page questionnaire. The response rate was compared with that obtained from the first postal wave in the 1991-1992 survey. Although the response rate was unchanged when using the one-page questionnaire (45% versus 45%), it decreased by 7% when the two-page questionnaire was used (38% versus 45%). On the basis of these results, no problem should arise if four more questions are added to the one-page questionnaire. The slight reduction in the response rate of the two-page questionnaire is worrying but could be corrected by the use of telephone interviews.