Allergen specific immunotherapy (IT) has been widely used for many years as a specific treatment of allergic diseases. A variety of changes in immunological parameters have been described but it still remains uncertain as to which of them is responsible for the improvement of symptoms. The aim of our study was to evaluate the effect of IT on natural killer (NK) cell activity, IL-4, IFN-gamma, IgE levels and skin test reactivity in addition to clinical efficacy. Thirty-one patients with allergic rhinitis and asthma were selected according to positive history, skin prick tests to Dermatophagoides pteronyssinus (Der p) or grass pollens, presence of specific IgE antibodies in sera and clinical findings, and were submitted to one year of placebo-controlled IT. Total IgE, specific IgE, IL-4 and IFN-gamma levels were measured by using ELISA method. Standard chromium 51 release assay was used to measure NK cell cytotoxic activity against the human leukemic cell line, K562 target cells. Mean symptom and medication scores, skin test reactivity and histamine sensitivity were significantly decreased in the patients given IT at the end of the first year when compared with the placebo group. However, there was neither a significant reduction in total and specific IgE levels nor a significant increase in IFN-gamma levels at the first year of IT. IL-4 levels were only measured at the beginning of the study because of the very low levels. A decrease in NK cell activity was found in patients treated with grass pollen extracts after 12 months when compared with Der p and placebo group. No signs of major local or systemic side effects due to IT were seen in patients during the study. Although significant clinical efficacy of specific IT with standardized extracts has been demonstrated in allergic rhinitis and asthmatic patients at the end of the first year of IT, no significant changes in immunological parameters were observed. However we conclude that a decrease in NK cell cytotoxic activity during IT has to be taken into account in the follow-up of patients.