A new method for the intraoperative diagnosis of a perilymphatic fistula is presented. Two cases are reported in which the test clearly indicated the presence of a fistula even though no visible leak of fluid was noted by the surgeon. In each case, there was a clear history of vertigo and hearing loss. Preoperatively, the presence of a perilymphatic fistula was suggested using raised intrathoracic pressure test in electrocochleography (ECochG). During the surgery, there was no visible leak of fluid into the middle ear. Electrocochleography was undertaken intraoperatively by placing an electrode in the round window niche and suctioning the oval window area; in neither case was any ECochG change seen. Next the electrode was repositioned on the oval window and when the round window area was suctioned, obvious ECochG changes occurred. Action potential (AP) amplitude dramatically decreased in both cases and summating potential (SP) increased in one case.