We studied the mast cells and the T-lymphocyte subpopulations of four patients' nasal polyps using immunohistochemical procedures. The data showed that MC displayed IL2-receptor and that they were very often closely related to CD4+ and MHC class II-positive T-lymphocytes. CD8-positive T-cells represented only 10% to 15% of the whole lymphoid population and were not localized near mast cells. These findings suggest a possible role of cell-mediated immune reactions in the pathogenesis of nasal polyps.