The purpose of this research is to prospectively evaluated the relationship between the intensity of airborne allergen exposure and the development of allergic sensitivity (production of allergen specific IgE) in a cohort of 80 children followed from birth to four years of age. The children selected for study will have a high genetic risk of atopy based on an elevated cord blood IgE concentration of greater than or equal to 0.6 u/ml. The quantities of the five inhalant allergens, to which young children most commonly become sensitized, will be measured in monthly air samples from each child's bedroom using a small volumetric air sampler, and an ELISA inhibition assay. The allergens selected are: cat, dog mite, ragweed, and timothery. Other variables thought to be related to the risk of atopy; tobacco smoke exposure, feeding history, and respiratory infections, will also be studied to determine whether any or these variables contribute more to the risk of inhalant allergy than intensity of allergen exposure. Tobacco smoke exposure will be quantitated by measuring urinary cotinine concentrations every two months. The child's feeding history and history of respiratory illnesses will be recorded during monthly interviews in a standardized fashion. The onset and degree of allergic sensitization will be determined by measuring serum concentrations of allergen specific IgE, to the same five allergens every six months. The data collected will be analyzed to determine if the intensity of allergen exposure is the most important risk factor for the development of allergic sensitization in a genetically susceptible population. We will also examine whether there is a critical intensity of exposure below which sensitization is unlikely to occur and whether the intensity of exposure and degree of sensitization are correlated. The answers to these questions should suggest the degree to which environmental control measures could reduced the risk of allergic sensitization. This information is important in the effort to reduce the rising prevalence of asthma and other allergic diseases among children.