Background: Over the past decades the prevalence of asthma has increased in most parts of the world, and widespread changes in lifestyle and environment have been postulated as the primary cause for this. We examined whether the extent to which genetic and environmental factors influence susceptibility to asthma has changed between 1994 and 2003. Methods: Two different cohorts of twins (n = 3393 and n = 2813 pairs), both 12-20 years of age, from the Danish Twin Registry, participated in questionnaire studies nine years apart. Prevalence of asthma and resemblance between twins for asthma was compared between the two cohorts. Results: The prevalence of asthma increased from 7.1% in 1994 to 10.8% in 2003, p < 0.001. The change was significant both in males (7.7 vs. 12.4%, p < 0.001); and females (6.6 vs. 9.2%, p < 0.001). The best-fitting model, which included additive genetic and non-shared environmental effects, showed that the heritability of asthma increased significantly from 79% in 1994 to 91% in 2003, p = 0.005. Conclusions: This result could indicate that environmental changes over the past years have increased the expression of genes and thus have lead to a higher heritability in the succeeding cohort. This is consistent with the hypothesis that changes in the environment, particular in childhood, are the cause of the increase in the prevalence of asthma over the past decades. © 2011 Elsevier Ltd. All rights reserved.