Forty-one patients suffering from grass pollen allergy underwent specific immunotherapy with standardized allergen extract consisting of six grass pollens (H-Al per os) administered either sublingually or supralingually for one year. In order to investigate clinical and immunological changes induced by the administration of allergens via the oral mucosa, the double-blind, placebo-controlled, randomized design of the trial with 30 other patients enrolled in placebo groups was applied. Specific immunotherapy with oral drops administered sublingually or supralingually was performed in the same way, keeping the drops under or on the tongue, respectively, for 1-2 min before swallowing them; at the end of the trial the cumulative dose of the allergen was almost 20 times higher than that of the subcutaneous therapy with corresponding allergen preparation. Data about symptoms scores and drugs intake during grass pollen season, as well as skin reactivity, levels of specific IgG and IgE antibodies, before the study and after the study's completion, were obtained. It was found that both routes of administration are effective according to subjective clinical parameters and drug consumption, with a highly significant reduction of symptoms and drug intake favoring sublingual administration where a reduction of more than 60% was achieved. Only sublingual active group showed a significant increase in Dactylis glomerata-specific IgG serum levels. Adverse effects were limited to a small number of generally mild local and/or systemic reactions. The results suggest that the administration of allergens via the oral mucosa is safe and clinically effective, favoring the sublingual rather than supralingual route.