20 patients with sudden deafness were examined by electrocochleography (ECochG). The cochlear potentials such as AP, CM and SP were investigated both at the early stage of the disease and 2 to 4 weeks later. There was a significant difference in the cochlear potentials between the cases with remarkable hearing improvement and with improvement. In all cases whose SP/AP ratio was more than 0. 27, hearing improvement was obtained. The SP/AP ratio returned normal (about 0. 25), after the hearing recovery. It was concluded that the SP/AP ratio in ECochE might be very helpful in studying the prognosis of sudden deafness.