Objectives: To evaluate whether a reactivation of Herpes virus (HSV) affect the prognosis in idiopathic sudden sensorineural hearing loss and whether the antiviral medication in patients with sero-postivie of HSV IgM would be effective for the treatment. Methods:A retrospective study of 232 patients with idiopathic sudden sensorineural hearing loss was performed for serologic virology tests, hearing thresholds, prognosis, and treatment. HSV type 1 and type 2 reactivation had been confirmed by polymerase chain reaction or serologic assay. The two groups with sero-positive and sero-negative IgM were compared for the prognosis of disease. The treatment (antiviral medication, steroid medication, and intratympanic steroid injection) were evaluated in patients with sero- postive IgM. Result:HSV IgM was 14.2% in patients with idiopathic sudden sensorineural hearing loss (HSV1 IgG=45.7, HSV2 IgG=19.4%, VZV IgM=4.3%). There were no significant differences of initial hearing, first recovered time, final recovered hearing, recovery duration between sero-postive and sero-neagtive group. The patients with anti viral medication did not show better prognosis than the patients without the treatment (p>0.05). The correlations between herpes virus titer and hearing was not significant (R2=0.024, p=0.109). Conclusion:The evaluation of viral titers for diagnosis of idiopathic sudden sensorineural hearing loss would not be useful to expect the prognosis. The anti viral treatment for patients with sero-positive IgM did not also affect the better prognosis of hearing improvement.