Hyposensitization (desensitization, immunotherapy) is--next to avoidance of allergens--the only causal treatment of IgE-mediated allergic diseases. In a number of well-controlled studies, immunotherapy has shown a clinical efficacy in allergic rhinitis, mainly hay fever, and in allergic asthma. Here, immunotherapy is primarily useful in children and young adults. Immunotherapy is highly effective in hymenoptera-venom allergy. The quality of the extracts in terms of potency and composition is essential both to the diagnosis of relevant allergens and to an optimal result of immunotherapy. The hyposensitization extracts used in Switzerland are listed in the 'Spezialitätenliste des Bundesamtes für Sozialversicherung' and have, therefore, to be payed for by the health insurance. Since the early seventies so-called semi-depot extracts are used in practice; here, the allergens are adsorbed to aluminium hydroxide and thus liberated in a delayed way. For a few years a short-term therapy, using so-called depot allergoids, is possible. A hyposensitization should be initiated only after a through evaluation and a careful allergological examination. With regard to pollen allergies one has to take into consideration especially the presence of the leading pollens, the cross-reactivity and the patient's account of his symptoms. The practice of hyposensitization is discussed extensively. The information of the patient regarding efficacy, length of treatment and risk of side effects is of great importance.