Atopic dermatitis is a multifactorial disease that seems both to rise in frequency and to be dependent on a genetic predisposition. In order to clarify these issues we encircled a representative twin series with atopic dermatitis from a total twin population of 592 like-sexed twin pairs. We found that the cumulative incidence rate (0-7 years) of atopic dermatitis in Denmark has increased significantly from 0.03 for the birth cohort 1960-1964 to 0.10 for the birth cohort 1970-1974, that monozygotic twin pairs are more often concordant for atopic dermatitis than dizygotic twin pairs, that monozygotic twins run a risk of 0.86 of having atopic dermatitis if the twin partner has the disease, whereas the disease risk of 0.21 run by dizygotic partners does not differ from the frequency seen in ordinary brothers and sisters. The results indicate that genetic factors play a decisive role in the development of atopic dermatitis and that widespread environmental factors are operating in genetically susceptible individuals.