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The objective of this study was to investigate the relationship between endolymphatic hydrops (EH) and hearing level or vestibular symptoms by focusing on EH herniation into the semicircular canal (SCC).Retrospective study.University hospital.The study included 1,548 ears of 775 patients who underwent magnetic resonance imaging (MRI) examination in our university hospital to investigate possible EH. MRI was performed 4 hours after intravenous injection of a standard dose of gadodiamide hydrate and/or 24 hours after intratympanic injection of gadopentetate dimeglumine diluted eightfold. The hearing threshold and vestibular symptoms were compared between ears with unilateral and bilateral EH herniation into the SCC and between ears having vestibular EH adjacent to the stapes footplate with or without EH herniation.Forty-four ears (19 men and 25 women, mean age 53.6 yr) showed EH herniation into the SCC. The average hearing thresholds at 500 to 4000 Hz and presence of vestibular EH adjacent to the stapes footplate were significantly higher in ears with unilateral herniation than in those with bilateral herniation. The average hearing thresholds at 500 and 1000 Hz were significantly higher in the group of ears having adjacency with herniation than in those without herniation. Vestibular symptoms did not differ significantly between groups.Unilateral herniation occurs with EH progression. Bilateral herniation may occur regardless of EH progression and might be influenced by other factors that alter the membranous labyrinth.