Idiopathic sudden sensorineural hearing loss(ISHL) is and abrupt onset of sensorineural hearing loss over a period of hours or a few days without definite cause. Like any acute sensory deprivation, it demands an aggressive diagnostic and therapeutic approach which are based on known and proposed etiologies and pathophysiology of ISHL. However, the causes, prognosis and treatment of ISHL have not been determined with precision until recently. While many treatment regimens have been proposed for ISHL, Ugo Fisch(1983) showed that carbogen inhalation yielded significantly better hearing results than intravenous vasodilators and low-molecular-weight dextran, and recommended carbogen inhalation for an effective nonivasive treatment assisting the spontaneous recovery of ISHL. The purpose of this study is to evaluate the effect of carbogen inhalation on 21 patient with ISHL. The results were as follows : 1) The therapeutic results of the 21 cases were 6 cases(28.6%) of complete recovery, 6 cases(28.6%) of partial recovery, 1 case(4.8%) of slight recovery and 8 cases(38.1%) of no improvement. 2) Those patients who were treated within 2 weeks from onset revealed better favorable recovery rates. 3) The hearing recovery was not poor in cases with vertigo than those without vertigo. 4) Profound hearing loss on the initial audiogram had poor response. 5) The effect of carbogen inhalation was prominent in cases of cochlear lesion than in cases of unidentified site of lesion. 6) The side effects of carbogen inhalation were not significant. 7) Carbogen inhalation is also recommended for the relief of tinnitus.