We have observed histopathologically the mucous membrane of the maxillary sinus in 86 cases who received Caldwell-Luc's operation during four years period from Jan. 1977 to Dec. 1980 in National Police Hospital. The results were obtained as follows. 1) In the age incidence, the highest incidence was noticed between 20-24 years of age (35 cases, 40.7%). In Sex distribution, 75 cases (87.2%) were males and 11 cases (12.8%) were females, and the ratio between male and female was about 6.8 : 1. 2) The pathological classification of the mucous membrane revealed as follows; edematous type in 50 cases (58.1%), fibrous type in 22 cases (25.6%) and infiltrative type in 14 cases (16.3%). 3) The state of mucous gland of the mucous membrane revealed as follows; marked hyperplasia in 17 cases (19.8%), moderate hyperplasia in 37 cases (43.0%) and normal or atrophy in 32 cases (37.2%). 4) The changes of blood vessel of the mucous membrane revealed as follows; marked change in 21 cases (24.4%), moderate change in 25 cases (29.1%) and slight change in 40 cases (46.5%). 5) The grade of cell infiltration of the mucous membrane revealed as follows; marked infiltration in 10 cases (11.6%), moderate infiltration in 30 cases (34.9%) and slight infiltration in 46 cases (53.5%).