Background Asthma is a complex disease characterized by symptoms of wheezing, shortness of breath, chest tightness, and cough. Objective To study the relative contribution of genetic and environmental factors in the liability to asthma in a large sample of twins. Methods Data on asthma in 21 135 twin pairs, 3-71 years of age, from the Danish Twin Registry were collected via a multidisciplinary questionnaire survey. Heritability estimates were calculated using variance components models. Results A monozygotic twin had an approximately sixfold increased risk of asthma whereas a dizygotic twin only had an approximately threefold increased risk relative to the general population if his or her co-twin was affected. The difference was more pronounced among males. Familial aggregation of asthma in children and adolescents was explained mainly by additive genetic factors, but common environment was also important. The heritability of asthma was also substantial in adults aged 20-49 years. In older adults (aged 50-71 years), additive genetic factors did not significantly influence the disease risk. Conclusion Genetic influences on asthma are substantial throughout the life span but the proportion of the disease liability explained by genetic factors is decreased in older adults.