BACKGROUND: Persons with atopic dermatitis or eczema, regardless of disease severity or activity, may develop eczema vaccinatum if they or their close contacts receive the smallpox vaccine. According to current recommendations, a preexposure vaccination program should identify these persons and exclude them from participating. OBJECTIVE: To determine the prevalence of diagnosed atopic dermatitis and eczema in a defined population and assess the sensitivity of screening questions to identify patients who have received these diagnoses. DESIGN: Population-based prevalence survey and telephone interview. SETTING: 14 ZIP code regions in Wisconsin. PATIENTS: Persons given a diagnosis of atopic dermatitis or eczema in 2000 and 2001 were identified from a population-based cohort. Persons with a history of atopic dermatitis diagnosed since 1979 were eligible for the telephone survey. MEASUREMENTS: Prevalence of diagnosed atopic dermatitis or eczema; proportions of respondents able to recall a past diagnosis of atopic dermatitis, eczema, or recurrent rash. RESULTS: The prevalence of atopic dermatitis or eczema diagnosis in 2000 or 2001 was 0.8%. At least 2.4% of the cohort would be ineligible for smallpox vaccination because of active skin disease in themselves or household members. Among 94 adult respondents with atopic dermatitis, 55 (59%) correctly self-reported skin disease. Seventy-nine (60%) of 133 household contacts of adults with atopic dermatitis correctly reported the presence of skin disease in a household member. Parental recall of skin disease in children with atopic dermatitis was 70% (123 of 177). CONCLUSIONS: Identifying dermatologic contraindications to smallpox vaccination by relying only on a self-reported history of rash illnesses is likely to miss a substantial proportion of individuals who should not receive smallpox vaccine in a preexposure vaccination campaign.