The aim of this study was to estimate the relative influence of genetic and environmental factors on the aetiology of asthma. The classic twin study design was used to analyse data on self-reported asthma obtained by a questionnaire mailed to 34,076 individuals, aged 12-41 yrs and originating from the Danish Twin Register. The cumulative incidence of asthma was 6.2% in the pooled sample. The data showed a decreasing cumulative incidence with age for males, but no sex or zygosity difference was observed. Substantially higher concordance rates, odds ratios and correlations for asthma were estimated in the monozygotic than in the dizygotic twins. Using biometric modelling, a model including additive genetic and nonshared environmental effects provided the best overall fit to the data. According to this model, 73% of the variation in liability to asthma was explained by genetic factors. No sex difference or age-dependency in the magnitude of genetic effects was observed. The biometric analysis emphasized a major influence of genetic factors in the aetiology of asthma. However, a substantial part of the variation in liability to asthma is due to the impact of environmental factors specific to the individual. There is no evidence for a substantial impact of genetic dominance or the shared environment.