BACKGROUND: It is the general impression that the prevalence of asthma has increased during recent decades. A study was undertaken to investigate asthma prevalence, respiratory symptoms, and lung function in young adults in the City of Copenhagen 15 years apart. METHODS: Men and women aged 20-35 years were sampled from the general population living in a defined area of central Copenhagen. The first examination took place in 1976-8 and comprised 1034 subjects (response rate 67.2%). A new sample comprising 1104 subjects (response rate 62.6%) from exactly the same area was examined 15 years later in 1991-4. All participants answered a questionnaire on respiratory symptoms and diseases and performed spirometric tests with measurement of forced expiratory volume in one second (FEV(1)) and forced vital capacity (FVC). RESULTS: The prevalence of self-reported asthma increased from 1.5% in the first survey to 4.8% in the second survey (p<0.001). Asthmatic subjects had, on average, poorer lung function than non-asthmatic subjects in terms of FEV(1) and this difference was more pronounced in the second survey than in the first (10.0% of predicted versus 2.4% of predicted). Smoking decreased significantly from 62% in 1976-8 to 45% in 1991-4 (p<0.001). CONCLUSIONS: The prevalence of self-reported asthma has increased significantly among young adults in Copenhagen over a 15 year period. The severity of asthma, as judged by the level of FEV(1), has also increased. These findings cannot be explained by changes in smoking habits.