The aim of this prospective single-blind randomized controlled study was to evaluate the therapeutic efficacy of high dose intravenous vitamin C (HDVC) added to systemic steroid in patients with idiopathic sudden sensorineural hearing loss (ISSNHL). Between August 2010 and August 2011, 72 ISSNHL patients who participated in this study were randomly allocated to two groups: 36 to a control group, members of which were given systemic steroid treatment for 15 days, and 36 to a HDVC group, members of which were given HDVC (200 mg/kg/day) for 10 days in addition to steroid therapy followed by oral vitamin C (2,000 mg) for 30 days after discharge. Finally, we analyzed each group: 35 as a control group and 32 as a HDVC group. Auditory evaluations were performed by pure tone audiometry (PTA) before and ~1 month after treatment using Siegel's criteria. HDVC group showed significantly greater complete and partial recovery improvement (p = 0.035). In addition, the complete recovery rate in the HDVC group was more than twice that of the control group (p = 0.031). In the HDVC group, PTA improved from 67.6 ± 19.8 dB HL before treatment to 37.1 ± 28.8 dB HL at 1 month after treatment, whereas in the control group, PTA improved from 70.3 ± 12.4 to 47.6 ± 25.2 dB HL, which represented a significant intergroup difference (p = 0.030). In conclusion, HDVC may enhance hearing recovery in ISSNHL patients, which suggests that HDVC reduces levels of reactive oxygen metabolites produced by inner ear ischemia or inflammation, and that HDVC could be considered for the treatment of ISSNHL.