We investigated 24 cases of idiopathic sudden progressive deafness (ISPD) in which worsening of sensorineural hearing loss after initial examination was confirmed by audiometry. Exploratory tympanotomy was performed in 10 of 24 patients, and revealed perilymphatic leak in 4 patients. However, there was a higher incidence and a more severe degree of vertigo and imbalance in the group that was operated on than in the non-operated group. Our study has shown that there was no clear difference in the degree of hearing improvement between the operated group and the conservatively treated group. In both of these groups, an abruptly falling audiometric configuration was frequently found in the audiogram obtained at the stage of fixed hearing perception, suggesting the possibility that both groups may have a common lesion at the basal turn of the cochlea.