A subclinical viral labyrinthitis has been postulated in the literature to elicit Idiopathic Sudden Sensorineural Hearing Loss. An etiological role for the herpes virus family is assumed. Corticosteroids possess a limited beneficial effect on hearing recovery in ISSHL. In this study, the therapeutic value of the antiherpetic drug aciclovir (Zovirax) on hearing recovery in 44 ISSHL patients receiving prednisolone is evaluated in a multicentre clinical trial. The study is designed prospectively, randomized, double-blind and placebo-controlled. Subjective parameters include hearing recovery, a pressure sensation on the affected ear, disequilibrium or vertigo and tinnitus. Audiometric parameters include pure tone and speech audiometry. A one-year follow up is obtained. Both the pressure sensation and disequilibrium or vertigo have a good prognosis, but tinnitus, occurring in most patients, has a poor prognosis. Hearing recovery prognosis depends on the severity of initial hearing loss, and not on vestibular involvement. No beneficial effect from combining aciclovir with prednisolone can be established in ISSHL.