Asthma is a major chronic disease in the U.S. affecting 20 million people and costing 6.2 billion dollars per year. The prevalence and severity of asthma in children is increasing, with disproportional morbidity and mortality among Hispanic and African-Americans. Air pollutants and indoor allergen are hypothesized to be major risk factors in development and exacerbation of asthma in children. Our knowledge of the impact of these environmental agents on asthma is derived from limited cross-sectional studies. In this prospective cohort study we test the hypothesis that carefully quantitated indoor allergen exposures (including house dust mite, dog, cat and, cockroach antigen, and fungi) and air contaminant exposures (environmental tobacco smoke, nitrogen dioxide, nitrous acid, ozone, fine particle mass, fine particle mass composition and pollen) are associated with the incidence and severity of asthma in children whose health status, familial history, antigen-specific IgE status, home environment, and potential confounders are carefully characterized. Emphasis is placed on a study design where the results will be applicable to the general population and will address the increase in asthma morbidity in minorities. The study population is selected via a screening questionnaire administered to mothers delivering newborns at four area hospitals. A total of 1,000 mothers who have healthy infants and also have a child age 3 - 7 diagnosed with asthma by a physician will be selected for inclusion in the study. After an initial risk factors questionnaire (parental history of asthma, birthweight, pollutant sources, and SES), the siblings will be followed for one year for asthma severity (frequency of ER visits, medication usage, and activity limitations) and concurrent exposures to indoor aeroallergens and air contaminants. During the summer months diurnal peak flow differences and daily medication use for 500 siblings classified as mild or moderate asthmatics will be recorded and compared with daily measured ambient air contaminant concentrations. The infants will be followed on a tri-monthly basis for four years to assess asthma development as measured by a physician diagnosis, bronchodilator use or episodes of wheeze. A nested exposure assessment protocol will be used and potential confounding factors will be controlled using multivariate analysis. This study will provide a basis for prioritizing environmental interventions to reduce asthma morbidity in children.