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Determine the prevalence and time interval for conversion from unilateral to bilateral involvement in Meniere's disease and cochlear hydrops.Retrospective chart review in a tertiary otologic referral center.232 patients diagnosed with Meniere's Disease (n = 186) or cochlear hydrops (n = 46) between 1959 and 2001, who visited the clinic over a five-year period between 1997-2001 and have at least 2 audiograms more than 12 months apart.Prevalence of cochlear hydrops relative to Meniere's Disease, rate of progression from unilateral to bilateral involvement; interval between unilateral onset of symptoms and bilateral involvement; and rate of progression from cochlear hydrops to Meniere's disease.Initial diagnosis was Meniere's disease in 71% and cochlear hydrops in 29% of all 950 hydropic patients presenting between 1997 and 2001. In the study sample, Meniere's disease was bilateral at presentation in 11%; an additional 12% (14% of unilaterals) became bilateral during the follow-up period. At presentation, 6.5% of cochlear hydrops patients were bilateral, with another 26% becoming bilateral. Conversion from cochlear hydrops to Meniere's disease occurred in 33% and some of these are included among the bilateral. The average time interval for conversion from unilateral to bilateral Meniere's was 7.6 years (SD = 7.0 years).Most otologists are aware of the potential for contralateral ear involvement and conversion from cochlear hydrops to Meniere's disease after diagnosis. These changes are significant, require long-term follow-up for detection, and may necessitate further treatment. Patients should be counseled regarding this potential when interventions are considered, especially with respect to ablative treatments.