The degree of the temporal bone pneumatization is various and it's development and function have not been clearly explained as yet. The range of normal hearing threshold level which is measured by pure-tone audiometry is variable (-10~25dB. ISO, 1964) because of sensitivity of the auditory mechanism, psychological factor, location of the bone vibrator, the anatomical conditions of the mastoidal region and so on. To evaluate the effect of pneumatization of the temporal bone upon hearing, authors analyzed 122 ears without previous and present illness in ears who had visited the Seoul National University Hospital during a period of 14 months from January 1979 till February 1980. The size of pneumatic cells of the temporal bone (taken by Law's view) was measured with a planimeter and the hearing threshold level with a pure tone audiometer and then the correlation between pneumatization degree and air-bone gap was investigated. The results were as follows : 1) The air-bone gap revealed a linear decrease in proportion to degree of the temporal bone pneumatization. 2) The correlation coefficients between the pneumatization of the temporal bone and air-bone gap were -0.795 at 500Hz,-0.717 at 1,000Hz, -0.763 at 2,000Hz and 0.793 at overall speech frequencies (P<0.01). 3) The above results suggested that the air-bone gap is small in the group of well penumatized temporal bones and large in the group of poorly pneumatized temporal bones.