The way nasal polyps arise and why they tend to recur is still unknown. Quite frequently they are found in association with asthma, rhinitis and ASA-intolerance, thus suggesting a multifactorial etiopathogenesis. The incidence of atopy in patients affected with nasal polyposis is quite low (16.8%). Recent studies stress the involvement of mast cell mediators due to various degranulating stimuli other than those mediated by IgE. The finding of the interleukin-2 receptor (IL2-R) on murine mast cells and on human peripheral blood basophils, together with the possibility of inducing basophil degranulation through IL2 stimulation, have led the authors to seek IL2R on human nasal polyp mast cells and to study subpopulations of nasal polyp lymphoid infiltrates. Nasal polyps obtained from 4 patients, admitted to the E.N.T. Department of the Catholic University of Rome in 1988, were snap frozen soon after their surgical removal through transmaxillary ethmoidectomy. In this study the following monoclonal antibodies (MoAb) were used: Leu-2a (CD8), Leu-3a/3b (CD4), Leu-4 (CD3), anti-HLA-DR and anti-IL2-R (CD25), OKM1 (CD11), OKB2 (CD24) and 1HT4-4H3 (CD 25). In no patient was there evidence of atopy, asthma or ASA-intolerance. Several mast cells (MC) were observed, chiefly in the connective axis and perivascular areas. These cells were characterized by a large number of cytoplasmatic monomorphic granules. The MC displayed the IL2-R and they were very often close to T-lymphocytes. T-cell subpopulations were predominantly composed of CD4-positive cells (about 75% of all lymphocytes) often associated in clusters and located both in the submucosa and in the connective axis. CD8-positive cells (10-15% of the lymphoid cells) were located most often just under the epithelium. They were hardly ever scattered within the CD4-positive cell clusters. Almost all T cells were activated, above all those surrounding the MC. These results would appear to suggest the presence of a cell-mediated immune response in nasal polyp pathogenesis where MC degranulation, determined by activated T-cell cytokines, plays an important role.