OBJECTIVE Unintentional electrical stimulation of the facial nerve by cochlear implants occurs when advanced otosclerosis invades the endosteum of both the upper basal turn of the cochlea (UBTC) and the facial nerve canal (FNC) and all the bone between these 2 structures. BACKGROUND A complication of cochlear implantation is facial nerve stimulation (FNS) known to be more common in otosclerosis. Otosclerotic involvement of the enchondral bone of the otic capsule results in areas of bone resorption, new bone formation, vascular proliferation, and a connective tissue stroma. This may reduce impedance, shunting current to the facial nerve. The cause of FNS has not been fully elucidated, and remarkable differences in FNS rates have been reported using different types of electrode arrays. METHODS Thirteen implanted temporal bones from 11 patients with otosclerosis, 10 with straight, and 3 with perimodiolar electrodes, were histologically processed after death. The data were analyzed using Fisher's exact test. RESULTS In the straight electrode group (n = 10), only those subjects with temporal bones showing involvement by otosclerosis of the UBTC and of FNC endosteum and the bone between these 2 structures (n = 4; 40%) showed FNS during life (p = 0.005), which was consistent with the location of problematic electrodes during life. None of the cases in the perimodiolar group had FNS even with endosteal involvement by otosclerosis. CONCLUSIONS FNS is a common complication of cochlear implantation in patients with otosclerosis and occurs most commonly with straight electrode implants where the endosteum of both UBTC and FNC and the intervening bone are otosclerotic.