The susceptibility to develop atopic dermatitis can be attributed both to genetic and environmental causes. We estimated the relative impact of genetic and environmental factors in the etiology of atopic dermatitis in a population-based sample of twins. From the birth cohorts of 1953-1982 who were enrolled in The Danish Twin Registry, a total of 11,515 twin pairs were identified in a nationwide questionnaire survey. Subjects were classified as atopic dermatitis cases when responding affirmatively to the question, "Do you have, or have you ever had, eczema in the folds of your elbows or knees?" Latent factor models of genetic and environmental influences were fitted to the observed data using maximum likelihood methods. The overall lifetime prevalence of atopic dermatitis was 7.3%. A cotwin of an affected identical twin had a sevenfold increased risk of atopic dermatitis compared with a threefold increased risk among cotwins of an affected fraternal twin, relative to the general population. Genes accounted for 82% and nonshared environmental factors accounted for 18% of the individual susceptibility to develop atopic dermatitis. The same genes contributed to the susceptibility to atopic dermatitis both in male and female patients (p = 0.98). The estimates were adjusted for age. The susceptibility to develop atopic dermatitis is attributable to mainly genetic differences between people. However, differences in environmental exposures also are of importance.