The aim of the present study was to describe the prevalence and severity of asthma in young Danish adults over three decades. Males and females aged 20-35 yrs were sampled from the population of Copenhagen for the three surveys (1976-1978, 1991-1993 and 2001-2004). A total of 3,285 (46% male) subjects answered a questionnaire, and had their height, weight, forced expiratory volume in 1 s (FEV1) and forced vital capacity measured. The prevalence of self-reported asthma was 1.5, 4.7 and 6.9%, respectively, in the three surveys (p<0.001). An increasing prevalence of asthma was observed in both males and females, although it was highest among females. The difference in FEV1 between asthmatic and nonasthmatic subjects gradually increased, being 2.3 (p = 0.56) and 14.2% of the predicted value (p<0.001), respectively, in 1976-1978 and 2001-2004. From the 1991-1994 survey, increasing body mass index, especially >30 kg.m(-2), was associated with a lower percentage predicted FEV1 (p< or =0.005), and further analyses suggested an additive effect of asthma and obesity on FEV1. The proportion of smokers declined from 60 to 38% (p<0.001). The prevalence and severity of asthma have continued to increase over the last three decades among young Danish adults, and the observed increase in severity seems, at least partly, to be related to the increase in prevalence of obesity.