Exposure to mouse allergen is a known cause of asthma in occupational settings and exhibits high prevalence and association with allergic sensitization in inner-city home environments. It has never been characterized on a nationally representative scale.This study was designed to characterize mouse allergen prevalence in a representative sample of US homes and to assess risk factors for increased concentrations.Allergen, questionnaire, and observational data were analyzed from the first National Survey of Lead and Allergens in Housing, a cross-sectional survey of 831 US housing units. Allergen levels were characterized and related to demographic factors and household characteristics.Detectable levels of mouse allergen (Mus m 1) exist in 82% of US homes. Kitchen floor concentrations exceed 1.6 microg/g, a level associated with increased sensitization rates, in 22% of homes. Increased concentrations (>1.6 microg/g) were observed in high-rise apartments and mobile homes, older homes, and low-income homes. Odds of having increased concentrations were increased when rodent (odds ratio [OR], 3.38) or cockroach (OR, 1.81) problems were reported and when floor mopping (OR, 2.17) was performed instead of vacuuming.Household mouse allergen is widespread in many settings at levels that might contribute to asthma morbidity. The likelihood of exposure can be assessed by consideration of demographic and household determinants.