The goal of this study was to assess the reliability of CT-Scan in the cases of conductive hearing losses with normal tympanic membrane. A computed tomography of the temporal bone (CT-Scan) has been performed in a prospective manner in all patients who underwent surgery for a conductive hearing loss with a normal tympanic membrane in our department. Out of 474 cases, 437 cases (92.2%) presented with otosclerosis. In 25 cases a minor malformation (5.3%) was found at surgery, and in 12 cases (2.5%), another diagnosis was made. Sensitivity of CT-Scan was 91.3% in otosclerosis and 57% in minor malformations. In 8.7% of cases, a superficial and beginning surgical focus was put in evidence whereas CT-Scan was normal. Theses cases represent infra-radiological cases of otosclerosis. In case of radiological otosclerosis, fenestral otosclerosis was found in 83.5% of the cases. CT-Scan was found specific on the operated and deaf side, but in 11.3% of the cases, a radiologic focus did not have a clinical consequence on the controlateral side. A radiological focus is not systematically responsible for a hearing loss. Finally, CT-Scan remains a reliable, sensitive and specific exam in the diagnosis of cases of conductive hearing losses with normal tympanic membrane.