The pure-tone thresholds of air conduction, airbone gaps and gains with prosthesis were investigated in 116 ears with chronic otitis media with perforation in order to clarify the indication and the diagnostic basis of the prosthesis test. The pure-tone thresholds of 65 ears were within 40dB. Sixty-one (95%) among them had no ossicular chain abnormality, and received type I tympanoplasty. In 44 of the 116 ears. the air-bone gaps were within 20dB. Ninty-eight percent of these ears had no ossicular chain abnormality and also received type I tympanoplasty. Thus, in these ears. the prosthesis test was not always necessary. Among 72 ears those air-bone gaps were beyond 20dB, 33 ears showed more than 10dB gains with prosthesis and 27 of these 33 ears received typy I tympanoplasty. Thus it can be said that a 10dB gain with prosthesis was a good indecation for the prospect of the middle ear pathology and the surgical method.