Most cases of otosclerosis show pathological changes only around the oval window. On the other hand, it is known that cavernous changes extending to the cochlea induce progressive sensorineural deafness. Three patients with severe bilateral deafness and severe cochlear otosclerosis as diagnosed by CT underwent cochlear implantation. The postoperative hearing abilities were satisfactory in two of the three patients at one year after the operation. However, one patient with advanced otosclerosis suffered from the complication of facial nerve stimulation two months later after the cochlear implantation. His available cochlear implant electrodes gradually decreased due to the facial nerve stimulation and at last the cochlear implant was no longer beneficial for reacquisition of his hearing ability. His CT densitometry revealed marked degradation of the CT value in the cochlear basal turn circumference in comparison with that in other patients showing good courses after the cochlear implantation. In conclusion, a cochlear implant operation is valuable in patients with severe sensorineural deafness with cochlear otosclerosis. However, we should cautious in performing cochlear implantation in patients with severe demineralization of the cochlear optic capsule.