Chronic rhinosinusitis (CRS) is a complex, multifactorial inflammatory disease of the nasal mucosa and paranasal sinuses, in which disruption of the integrity of the mucosal barrier initiates an inflammatory cascade. It results, among other things, in the formation of polyps, increased secretion and impaired sense of smell. The aim of the study was to investigate the effect of biological treatment with mepolizumab or dupilumab on the cytological examination of the nasal mucosa in CRSwNP patients. The study included 23 patients who underwent cytological examinations at least 3 times during 6 months of treatment. 21 patients were treated for 9 months. Samples were evaluated at 400x magnification and expressed as the mean of 50 high-power microscopic fields (HPF) that had been assessed. Ciliated cells, goblet cells, plasma cells, neutrophils, eosinophils and mast cells were counted. All patients had their sense of smell tested every 3 months using a visual analogue scale (VAS). During biological treatment of CRSwNP patients with mepolizumab and dupilumab, cytological examinations revealed an increase in the number of ciliated cells, a decrease in the number of eosinophils correlated with an improvement in the sense of smell on the VAS scale.