To investigate the implications of duration of deafness in the rehabilitation of unilateral deafness utilizing cochlear implantation. From the ongoing prospective cochlear implantation in unilateral deafness study, we looked at five adults who received a cochlear implant for long-term unilateral deafness. Speech perception in noise and subjective evaluation of the benefits of cochlear implantation were measured at 3, 6, and 12 months after implantation. The results were analyzed and compared with published data from normal hearing individuals and adults using cochlear implants bilaterally. Analysis of speech perception in noise showed significant improvement for three spatial configurations: speech and noise from the front (S0/N0; P=0.003), speech from the front and noise from the normal hearing ear (S0/NHE; P=0.001), speech from the implanted ear, and noise from the normal hearing ear (SCI/NHE; P<0.001). The scores obtained at 12 months after surgery improved to values similar to those obtained by individuals with normal hearing. The results of subjective measures showed significant improvement in hearing over time to the scores obtained by individuals with a bilateral cochlear implants and those with normal hearing. In this study, older adults with more than 25 years of unilateral deafness obtained scores in speech perception testing and in subjective evaluation that are similar to those attained by individuals with normal hearing and/or those with bilateral cochlear implants. Therefore, patients with postlingual unilateral deafness should not be excluded as cochlear implant candidates on the basis of a long duration of deafness.