The purpose of this study was to investigate the presence of endolymphatic hydrops (EH) in both affected and unaffected ears of patients with pantonal unilateral idiopathic sudden sensorineural hearing loss (ISSNHL) using three-dimensional fluid-attenuated inversion recovery magnetic resonance imaging (3D-FLAIR MRI) and further evaluate the significance of EH in this disorder. Twenty-seven ISSHL patients were enrolled in this study. 3D-FLAIR MRI was performed 24 h after intratympanic injection of gadolinium-diethylenetriaminepentaacetic acid (Gd-DPTA). The incidences of EH in the affected ears and contralateral unaffected ears were compared and the correlations of EH with vertigo or prognosis were analyzed using the Chi-square test. The results showed that the incidence of EH was 68.0% (17/25) in the affected ears and 34.8% (8/23) in the unaffected ears. There was a statistically significant difference between affected ears and unaffected ears in regard to the incidence of EH (P<0.05). There were no significant correlations of EH with vertigo (P=1.000) or with prognosis (P=0.359) in the affected ears. In conclusion, there is EH in the inner ear of patients with pantonal ISSNHL; EH is not related to vertigo, a concomitant symptom of ISSNHL, and the prognosis of this condition. The presence of EH may be a secondary reaction following the impairment of the inner ears with pantonal ISSNHL.