A histological study of 100 hearing ears showed that some capability for speech discrimination requires at least 10,000 spiral ganglion cells. The spiral ganglion cell populations were then estimated in another group of 62 ears which were profoundly deaf for a variety of causes and it was found that 45% of these met the criterion of having 10,000 ganglion cells. The ganglion cell populations were largest in ears deafened by sudden deafness, Meniere's Disease, and ototoxic drugs; they were somewhat less for those with vascular occlusion, temporal bone fracture, otosclerosis, and cochlear dysplasias; they were least in those with measles, bacterial labyrinthitis and congenital syphilis. The data is of relevance in the selection of patients for cochlear implantation.