Numerous studies have reported increases in asthma prevalence among children world-wide. Less is known about similar trends in adults. We aimed to investigate whether the prevalence of allergic asthma symptoms had increased in an adult general population. Two cross-sectional surveys using identical methods were carried out in 1989 and 1998. A one-page questionnaire on respiratory symptoms was mailed to random samples of 15-41-year-olds living in Copenhagen. The response rates were 86.6% (3624/4185) and 78.8% (2402/3048) in 1989 and 1998, respectively. The questionnaire was validated with specific immunoglobilin E (IgE) positivity as the reference in a random sample of responders in connection with both surveys. We found a significantly increased prevalence of subjects who reported shortness of breath on exposure to pollens (6.6% 0 s. 10.3%, odds ratio 1.61, 95% CI 1.34-1.95), furry animals (5.4% vs. 7.6%, odds ratio 1.45, 95% CI 1.17-1.79), and house dust (7.8% vs. 10.2%, odds ratio 1.35, 95% CI 1.12-1.61). The validation of these symptoms showed that the positive predictive values were reasonably stable over time, which may support that a true increase in allergic asthma has occurred. In conclusion, the prevalence of allergic asthma symptoms increased significantly in this adult general population over a 9-year period.