Specific allergen immunotherapy (IT) has long been used to treat allergic rhinitis and asthma. Review of the literature from the past 5 years continues to support this use of IT as the only disease-modifying treatment of allergic disease currently available. In addition, studies suggest that allergen IT may prevent the progression of allergic disease. In monosensitized patients, IT may prevent polysensitization. In younger patients with allergic rhinitis only, IT may prevent the new onset of asthma. Although these studies require further validation, the evidence is important enough to consider disease prevention as one of the indications for prescribing allergen IT.