Various atopic manifestations among adults have been shown to be influenced mainly by genetic factors. With the increase in prevalence of atopic diseases in recent years, especially among children, a great deal of attention has been given to environmental causes. In a study of 1480 Swedish twin pairs, 7-9 years old, we examined the importance of genetic and environmental factors in asthma, hay fever, eczema, and urticaria. Structural equation model fitting showed 33-76% of the variation in liability to the diseases to be due to genetic effects. Shared environmental effects were also important for hay fever and urticaria in both sexes and for eczema among girls. The clustering of atopic disease in families was almost entirely due to a common set of genes, but each disease manifestation also seemed to have specific genes of importance. Investigation of unlike-sex twins showed that boys had a higher cumulative incidence of asthma and hay fever than girls, whereas girls had a higher incidence of eczema. Thus, it may be concluded that although genetic factors are of major importance in atopic manifestation in children, both environmental and sex-related factors play a role.