In a series of 127 patients with idiopathic bilateral sensorineural hearing loss (IBSH) at the Hearing Clinic in the Department of Otolaryngology, Kitasato University Hospital, 20 cases showed rapid progression of hearing impairment. These patients had been treated with the same therapeutic regimen used in sudden deafness (S.D.). Improvement of hearing loss after treatment was generally less effective in the cases with rapidly progressed IBSH than in the cases with S.D. However, hearing improvement was achieved in some cases with rapidly progressed IBSH when the treatment had started in the early stages after the onset of rapidly progressive hearing loss. The mode of fluctuating hearing loss in the cases with rapidly progressed deafness was assumed to be similar to that in the group of all cases with IBSH.