Patients with diffuse obliterative otosclerosis have more extensive footplate pathology than annular cases. As a result of this more skill is required for diffuse otosclerosis cases, and postoperative hearing results are usually worse than annular cases. In this retrospective study we compared the preoperative audiological features of annular and diffuse otosclerosis patients. The subjects were 60 patients with conductive hearing loss who had undergone stapedectomy. Annular and diffuse groups were comprised of 30 patients each. Annular otosclerosis was defined as the footplate pathology involving the annular ligament only, where the footplate of the stapes is very thin and retains its bluish color. On the other hand diffuse, 0 bliterative otosclerosis was defined as the pathology involving the whole footplate and also in some cases extending beyond the confines of the annular ligament. In each group preoperative air- and bone-conduction levels at 125-6000 Hz and 500-4000 Hz were noted respectively. Average air-bone gap for the obliterative otosclerosis group was 37.5 dB; the same value for the annular group was 23.8 dB (p <0.05). The gap characteristics of the audiogram were different for the two groups. The annular group had an air-bone gap which was nearly constant for all the frequencies. In the diffuse otosclerosis group, the air-bone gap was more prominent in the low frequencies and it decreased at higher frequencies. No difference was noted in bone-conduction thresholds, and Carhart notch between the two groups. This study demonstrated that a large air-bone gap in patients with conductive hearing loss may be a sign of diffuse obliterative otosclerosis. This may warn the surgeon that a more challenging surgery is possible, and the patient may have a less favorable hearing result. Therefore, in the presence of a large air-bone gap, it may be appropriate to inform the patient of the strong possibility of diffuse otosclerosis.