Nasal polyps constitute a common pathology, but their etiology is not yet clear. In the present report, by means of immunohistochemical methods, both cell infiltrate and HLA-DR molecule expression were analysed in nasal mucous membrane fragments biopsied in the vicinity of fibrous polyps. Both surface and gland epithelia were positive for HLA-DR antigens in all sample whereas the epithelium of control nasal mucous membranes appeared to be negative. In addition only little delimited portions of the basement lamina seemed disrupted by an intense traffic of infiltrating cells. The poor venous drainage, the presence of activated mononuclear and mast cells capable of releasing soluble growth factors, together with basal lamina interruptions, might be factors for polyp sprouting.