A series of 13 patients where surgery confirmed retrocochlear lesions in the cerebellopontine angle was tested preoperatively by electro-cochleography. The following conclusions can be drawn: 1. Compound action potentials (CAP) of the acoustic nerve could be registered by electro-cochleography in all patients regardless of the fact whether hearing on the side of the lesion could be demonstrated in pure tone audiometry or not. 2. The threshold latency for the 2 kHz sinustone-burst elicited compound action potential was within the normal range if hearing on the side of the lesion could be shown in pure tone audiometry. Threshold latency for 2 kHz CAP was shortened pathologically when hearing on the side of the lesion was absent in pure tone audiometry.