We retrospectively evaluated the efficacy of combination therapy with steroid and hyperbaric oxygenation for sudden idiopathic sensorineural hearing loss (SISNHL). Patients (n: 109; 111 ears) visited our clinic within 14 days from onset before receiving treatment between January 1999 and March 2003. Hearing loss was assessed based on criteria prepared by the Ministry of Health and Welfare Acute Severe Hearing Loss Study Group. Patients were distributed into Group I-95 patients who started treatment within 7 days from onset-, and Group II-14 patients who started treatment within 8-14 days from onset. We evaluated the outcome of therapy using grading established by The Research Committee on Acute Profound Deafness, Ministry of Health and Welfare, Japan. The complete recovery of hearing was worse in patients with severe hearing loss. It was 4.8% in grade 4a, 18.2% in grade 3a, 25% in grade 2a, 20.0% in grade 4b, 38.5% in grade 3b, and 66.7% in grade 2b. We studied the relationship between type of hearing loss and recovery after treatment. The complete recovery of hearing was most favorable in patients with low tone hearing loss, followed by those with middle tone hearing loss and those with horizontal hearing loss. These findings indicate that the type of hearing loss was the most significant determinant of SISNHL prognosis and course. Twenty patients with acute stage SISNHL had diabetes mellitus. The recovery of hearing was almost the same in those with and without diabetes mellitus. Recovery was complete in 32.4%, Niarked in 32.4%, and slight in 21.6%. In 13.5%, no change was observed. Our results and data from previous reports, involving more than 70 Japanese patients treated with steroids alone, suggest that combination therapy with steroid and hyperbaric oxygenation is effective for SISNHL.