An objective method for the pre-operative diagnosis and the post-operative assessment of a presumed perilymphatic fistula (PLF) using transtympanic electrocochleography is presented. Three cases are reported in which the history of the disease and the symptoms strongly suggested the presence of a PLF. Pre-operative transtympanic electrocochleography (TT ECoG) recordings at rest showed changes similar to those of endolymphatic hydrops and signs of instability of the inner ear hydrodynamic system during raised intrathoracic pressure. Surgery revealed a visible leak in two of the three cases. Both windows were repaired in all the patients. All patients were relieved from their vestibular symptoms at the time when the post-operative TT ECoG was conducted 3-6 months later. The post-operative recordings were stable during raised intrathoracic pressure and the previously elevated summating potentials decreased which was interpreted as an objective indication of the recovery of the hydrodynamic system. However, later one of the patients again developed recurrent vertigo. Twenty patients with well-documented Ménière's disease were used as a control group. TT ECoG was conducted during raised intrathoracic pressure. The Ménière patients showed stable recordings. It is suggested that among patients with suspected PLF and signs of hydrops in TT ECoG, a dependence on the intrathoracic pressure reflected in the recordings may indicate a possible fistula.