Asthma can occur as a result of allergy to both house dust mites and to storage mites. Many people worldwide are exposed to more than one mite species in the domestic environment. The most common domestic mites areD. pteronyssinus, D. farinae, E. maynei, andB. tropicalis; other storage mites are found in homes much less frequently, even in tropical climates. Because of the ready availability of allergen extracts for skin testing and immunotherapy, patients with respiratory symptoms are generally evaluated for sensitization toD. pteronyssinus andD. farinae only. The evidence reviewed here suggests, however, that, although there are crossreactive allergens between many mite species, most species have specific allergens that can be an important cause of IgE responses. Therefore, the role ofB. tropicalis andE. maynei in the etiology of asthma merits further investigation. Identification of species-specific and crossreactive allergens and production of recombinant allergens would be useful tools in allergy practice. By skin-testing patients, physicians would then be able to evaluate whether patients recognize crossreactive allergens or species-specific allergens and whether or not there is a need for immunotherapy to more than one mite species.