Introduction. The results of recent metaanalyses showed the effectiveness of SLIT (sublingual immunotherapy) in allergic rhinitis and asthma in children allergic to grass pollen. However, in some meta-analyses high variability of the results of these trials weaken the power of conclusion. In the present study we used the highest dose of extract (of the ones registered in Poland). Aim of the study. To study the efficacy and safety of sublingual immunotherapy given pre-coseasonally (starting before the pollen season and continuing until the end of it) and its effect on inflammatory parameters in children with rhinitis allergic to grass pollen. Materials and methods. Thirty children, recruited from our outpatient allergy clinic, with rhinitis, sensitive to grass pollen, participated in the 2-year prospective, randomized, double-blind, placebo-controlled trial. All patients were randomized according to a computer-generated allocation schedule into the two treatment arms: the pre-coseasonal SLIT group (n= 15) (Staloral 300 IR, Stallergenes), and the placebo group (n=15). We analyzed the effect of SLIT on allergic rhinitis and ocular symptoms, the use of rescue medication, lung function, FENO and bronchial hyper-reactivity. Results. In the second year of immunotherapy we observed the reduction of nasal and ocular symptoms, significant decrease in the use of rescue medication and FeNO level as compared to the placebo group and to the first year of immunotherapy. We did not observe any significant changes in the lung function and bronchial hyper-reactivity after immunotherapy. Conclusions. The applied regimen of SLIT was effective and safe in children with rhinitis allergic to grass pollen. The results suggest that the measurement of FENO level is clinically useful in monitoring SLIT in children. [ABSTRACT FROM AUTHOR]