Infection and allergy are believed to play a role in the pathogenesis of nasal polyps. But, it is reported that nasal polyps are more frequent in patients without an IgE-mediated allergy than in patients with a proved IgE-mediated allergy. Presently, the role of infection and the subsequent inflammatory response, including release of cytokines, is regarded as the most likely cause of nasal polyps. This study was done to know the cellular types of immunocytes that were distributed through the tissues of nasal polyps in 24 patients and normal nasal tissues of 5 healthy persons by the immunohistochemistry using monoclonal antibodies specific for several immunocytes. The results are as follows : 1) There were much more infiltrations of lymphocytes in the tissues of nasal polyps than normal nasal tissues. 2) Most of the lymphocytes distributed in the nasal polyps were CD3-positive T cells. 3) Among the CD3⁺ T cells of nasal polyps, CD8⁺ suppressor/cytotoxic T cells were more than the CD4⁺ helper/inducer T cells. 4) Among the CD3⁺ T cells of nasal polyps, CD45RO⁺ memory T cells were more than the CD45RA⁺ naive T cells. 5) There were few CD20⁺ B cells scattered in the tissues of nasal polyps. 6) There were much more infiltrations of HLA-DR⁺ immunocytes in the tissues of nasal polyps than normal nasal tissues. There results suggest that immune-mediated inflammation could play the important roles in the pathogenesis of nasal polyps.