Objectives/hypothesis: The aim of our study was the objective assessment of endolymphatic hydrops in asymptomatic ears in patients with unilateral Ménière's disease, using a non-invasive electrophysiological tests and investigation of significant clinical signs. The null hypothesis was that there would be no signs of endolymphatic hydrops in the asymptomatic ear. Study design: For this study, we used the travelling wave velocity (TWV) test for endolymphatic hydrops. Methods: The TWV test was used in conjunction with standard audiological tests to investigate both ears of 181 Ménière's patients attending the MRC Institute of Hearing Research in Southampton. The test uses derived auditory brainstem responses (ABRs) to estimate the velocity of the cochlear travelling wave that is altered in endolymphatic hydrops. Ménière's disease was assessed using Arenberg's five staging criteria. Significant correlations were evaluated using standard statistical methods. Results: From 100 patients with clinically unilateral Ménière's disease, 27% showed evidence of endolymphatic hydrops in their asymptomatic ear. There was a significant correlation between signs of hydrops and the mean air conduction threshold at 500 Hz. Conclusions: We recommend that a full assessment of incipient disease in the asymptomatic ear of those suffering from unilateral Ménière's disease should be undertaken before offering any treatment options to patients. Any suspicion of early disease in the asymptomatic ear of patients with unilateral Ménière's disease should lead to full electrophysiological assessment to assess the evidence of endolymphatic hydrops in that ear.