There is evidence from epidemiologic studies, supported by more intensive study of selected groups of subjects, for the importance of allergy in initiating and contributing to the severity of bronchial asthma. Furthermore, removal of allergen exposure is followed by improvement in both symptoms and evidence of airway inflammation. Allergen immunotherapy reduces the sensitivity of the respiratory tract to allergens, blocks the influx of eosinophils and mucosal mast cells in response to allergen exposure, and alters the pattern of cytokine release by T-lymphocytes, generally decreasing Th2-related cytokines (IL-4) and increasing those related to the Th1 response (interferon-gamma, IL-2, IL-12). It would be remarkable, given these alterations in responsiveness produced by allergen immunotherapy, if this treatment were not effective in bronchial asthma. Indeed, an analysis of controlled studies of allergen immunotherapy does indicate that it is clinically effective in carefully selected, allergic asthmatics.