The cardinal principle of tympanoplasty is control of infection and hearing improvement but sensorineural hearing loss is occasionally developed after ear surgery. The postoperative hearing change is probably related to selection of patient, operation method, operation technique, graft materials and drilling during operation and so on. To evaluate the effect of drill-generated noise on the bone conduction threshold level, the bone conduction threshold shift after ear surgery was analysed with 50 cases who had ear operation with drill at the Department of Otolaryngology, S.N.U.H. during a period of 12 months from October 1979 to September 1980. The results were as follows ; 1) The noise levels were slightly influenced by the size and type of the burr. When drilling with suction irrigation were performed, louder noise was generated than when drilling on bone alone; the maximun noise levels is 103 dB(A) in the ipsilateral side and 95 dB(A) in the contralateral side. 2) In contralateral side, bone conduction threshold shift was 10 dB at 0.5 KHz, 15 dB at 1 KHz, 2 KHz and 4 KHz. 3) The above results suggested that drill-generated noise induced bone conduction threshold shift after ear surgery.