In 122 patients with sudden unilateral sensorineural hearing loss, no apparent cause was identified in 88 cases. In these idiopathic cases, gender did not affect the age distribution and age did not affect the range of most affected frequencies or the severity of hearing loss. Vertigo at onset of the disease was not a sign of poor prognosis. Cases in the idiopathic group could, generally, be distinguished into two groups: (1) cases with hearing loss affecting all (with one exception) or mainly the high frequencies and symptomatology reminiscent of known types of cranial (poly)neuritis; (2) cases with hearing loss affecting mainly the low frequencies and symptomatology reminiscent of Ménière's disease. These data suggest that there may be certain pathophysiologic similarities in these cases with: (a) cranial (poly)neuritis if all or mainly the high frequencies are affected; (b) Ménière's disease if the low frequencies are mainly affected.