Immunotherapy by the subcutaneous injection of increasing doses and then maintenance doses of extracts of inhalant allergens has been practiced for 100 years. Controlled clinical trials have established its efficacy in treating allergic rhinitis, asthma, and stinging insect sensitivity, and there are preliminary data to suggest a favorable response in some patients with atopic dermatitis. The response to subcutaneous injection immunotherapy is dose dependent. Disease-modifying actions include blocking development of new sensitivities in monosensitized patients, blocking progression to asthma in patients with allergic rhinitis, and persistence of treatment effects for up to 7 to 10 years after an initial course.