We have employed alpha-interferon (IFN-alpha), an anti-viral agent, in the treatment of severe idiopathic sudden sensorineural hearing loss (ISSHL). Forty-two patients were studied and had an average hearing ability of > or = 70 dB before treatment. We also examined 2'-5' oligoadenylate synthetase (2,5A-S) activity, one of the parameters indicating anti-viral activity of IFN, to investigate the relationship between the suppression of viral proliferation and prognosis and explain the pathogenesis of ISSHL. Complete recovery was found in 27 patients (64.3%) after IFN therapy. Increased 2,5A-S activity was observed on the 3rd day of IFN therapy in 24 of the 27 patients who completely recovered. No severe adverse events were reported after IFN therapy. Findings suggest that IFN therapy may be effective and safe in the treatment of ISSHL and calls for further investigation.