BACKGROUND:Atopic eczema onset is described primarily in early childhood; the frequency and characteristics of adult-onset disease remain controversial. OBJECTIVE:To determine the proportion of individuals who report atopic eczema symptoms between birth and mid adulthood, and to examine demographic, immunologic, and genetic factors associated with period of symptom onset. METHODS:We conducted a longitudinal study using data from two nationally representative community-based birth cohorts from the United Kingdom: the British Cohort Studies 1958 and 1970. Individuals were followed from birth through age 42-50. The primary outcome was the age period of self-reported atopic eczema symptom onset based on repeated measures of self-reported atopic eczema at each survey wave. RESULTS:The annual period prevalence of atopic eczema ranged from 5-15% in two cohorts of over 17,000 participants each followed from birth through mid-age. There was no clear trend in prevalence by age, and among adults reporting active atopic eczema during a given year, only 38% had symptom onset reported in childhood. When compared with individuals whose eczema started in childhood, those with adult-onset disease were more likely to be women, from Scotland or Northern England, of lower childhood socio-economic group, smokers in adulthood, and less likely to have a history of asthma. In a sub-analysis using data from the 1958 cohort only, genetic mutations previously associated with atopic eczema, including filaggrin null mutations, and allergen-specific IgE were more common among those with childhood-onset disease. CONCLUSION:Rates of self-reported atopic eczema remain high after childhood, and adult-onset atopic eczema has different risk factor associations than childhood-onset eczema.