The fall in incidence of chronic O.M. needing treatment indicates a reduction in the prevalance of active disease, but chronic O.M. is still a relatively common condition in children and an important cause of functional disability. During the period from June, 1976 to November, 1976, an epidemiological study of O.M.P.C. was done at five geographically different primary schools. Each school was located in Seoul city, Chung-Ju city, (a small city, Chung-Buk Province), poor farm area of Kang-Won, Kyung-Ki and Chung-Nam province. Subjects were 1596 children, aging from 9 to 13 years old. The followings are the results of this study 1) Incidence of O.M.P.C. in these school children was 1.69% in general, and in male it was 2.73%, 0.41% in female. 2) The ratio between unilateral O.M.P.C. and bilateral O.M.P.C. is about 2.4 : 1. 3) Among these different regions, the lowest incidence is in large city and the highest incidence is in rural area. 4) The factors which is a significant influence on O.M.P.C. are nutrients. 5) The main predisposing deseases of C.M.P.C. are measles and common cold. 6) There is a significant relationship between the size of T-M perforation and the duration of O.M.P.C.