Sublingual immunotherapy (SLIT) was developed to improve the safety of specific immunotherapy; however, its effectiveness is still subject to discussion although the balance sheet for SLIT is improving. In SLIT laboratory parameters and objective measures of allergen reactivity are nonuniform even in studies showing clinical effectiveness, thus subjective symptom scores remain the principal end points. For allergic rhinitis an expert panel collaborating with the WHO recently proposed that SLIT was a viable alternative for injectable immunotherapy (SIT) since a multitude of double-blind placebo-controlled studies had proved the effectiveness of SLIT. Unfortunately, there are only a small number of studies comparing effectiveness of SLIT directly with subcutaneous SIT. These studies demonstrated comparable effectiveness of both therapies. According to the data so far SLIT can be recommended for the therapy of allergic rhinitis in adults and children refusing injectable therapy. For the treatment of allergic asthma both positive and disappointing results have been published. Effectiveness in preventing the onset of allergic asthma in patients with allergic rhinitis has been demonstrated for SIT, while for SLIT this question cannot yet be answered. Copyright (C) 2003 S. Karger AG, Basel.