IN THIS issue of the ARCHIVES, Dr Tsitoura and her colleagues1 report on a remarkable international dataset of infants and young children at high risk for becoming allergic to, but not yet sensitized to, house dust mites. In a primary prevention trial, these children were randomized either to an intervention group that received a simple house dust mite–reduction strategy (mattress encasement and detailed preventive environmental recommendations, such as hot-washing of sheets and bedding) or to a control group, whose parents received information about general indoor environmental issues. After 1 year, children in the intervention group were less than half as likely as controls to have developed allergy to house dust mites (3.0% vs 6.5%). Furthermore, the frequency of asthma symptoms and asthma diagnoses was greater among the children who became sensitized to house dust mites during the study year. How potentially significant are these findings in the context of allergies and asthma? To consider this question, it is helpful to review selected issues focusing on the public health problem of asthma, which, unlike allergic rhinitis, frequently results in exacerbations that require hospitalization and other types of rescue care.