The ability of promontory testing (PT) to differentiate a retrocochlear from a cochlear lesion in a group of 88 patients having unilateral sensorineural hearing loss was assessed. Promontory stimulation was performed, using the Nucleus Promontory Stimulator (model Z10012, Cochlear Corporation, Melbourne, Australia), by placing a needle electrode transtympanically on the basal turn of the cochlea. Patients with measurable hearing gave inconsistent results. Patients who had undergone eighth nerve section or translabyrinthine removal of acoustic neuroma were used as controls. They had no perception of sound in response to electrical stimulation, and were unable to perform any part of the PT. Of nine patients with anacusis secondary to an acoustic neuroma, six were unable to perceive sound in response to electrical stimulation or perform any aspect of the PT. Of the three patients able to perceive sound, only one could perform the temporal difference limen (TDL) and gap detection (GAP) test. In patients with total deafness from other causes, 80 percent (24\30) were able to perceive sound with electrical stimulation, 46 percent (14\30) performed TDL, and 70 percent 21\30) performed the GAP tests. In patients with unilateral total deafness, promontory testing may aid in differentiating retrocochlear from cochlear lesions and help identify patients at risk for acoustic neuroma.