Sublingual immunotherapy has been suggested for the treatment of respiratory allergies, Many controversial studies have been reported on the efficacy of sublingual immunotherapy The aim of this prospective study was to evaluate whether sublingual immunotherapy was effective according to clinical and laboratory results in pediatric allergies. Thirty-nine allergic, grass pollen sensitive children were admitted into the study. Sublingual immunotherapy was given over a 12-month period to 21 children (mean age 10.5 +/- 3.3 years), 10 of whom had seasonal allergic rhinitis and II seasonal allergic asthma. During the same period, 18 children (mean age II. I +/- 2.5 years), 10 with seasonal allergic rhinitis and eight with seasonal allergic asthma, received placebo. Symptom scores and drug requirements were recorded and urine samples were collected to detect urinary levels of leukotrienes (Uc-LTB4 and Uc-LTE4). In patients who received sublingual immunotherapy, the symptom scores of seasonal allergic rhinitis significantly decreased but no statistically significant changes were observed in terms of symptoms of seasonal allergic asthma. Uc-LTE4 and Uc-LTB4 levels of seasonal allergic rhinitis, with a geometric mean and 95% confidence interval(CI), were significantly decreased from 216 (103-464) and 61 (22-198) pmol/mmol creatinine to 78 (29-159) and 35 (12-118) pmol/mmol creatinine, respectively (p < 0.05 and p < 0.05). On the other hand Uc-LTE4 and Uc-LTB4 levels for seasonal allergic asthma were 180 (92-355) and 78 (44-258) pmol/mmol creatinine and decreased to 156 (72-402) and 69 (32-254) pmol/mmol creatinine, respectively: These changes were not statistically significant (p > 0.05). According to our clinical results and urinary levels of leukotrienes, which are mediators showing the severity of allergic inflammation, it can be suggested that sublingual immunotherapy may be useful in the treatment of seasonal allergic rhinitis but not of seasonal allergic asthma.