The modern surgical treatment of otosclerosis consists of replacement of the sound conducting function of the stapes by a prosthesis. The results obtained in 100 consecutive patients using the small fenestra technique and a 0.4 mm. Teflon and steel wire prosthesis are reported. The surgical technique is described. The hearing was improved in all patients. In 92 per cent of the patients an optimal hearing gain was found after an observation time of 1 to 4 years. Five patients required re-operation during the observation time. The small fenestra technique and the Fisch prosthesis were considered optimal in respect to technical difficulty, hearing improvement and complication rate. There was no sensorineural hearing loss in this series of patients. The absence of serious complications makes it reasonable to operate on both ears in patients with bilateral hearing loss. The results are as good in elderly people as in younger people. Therefore the operation can be offered for patients in all age groups.