A sudden hearing loss (SHL) research clinic to which 30 members of an ENT Society refer patients, accumulates information about incidence, prognosis, recovery patterns, and treatment of SHL. The diagnostic protocol includes laboratory, audiometric, and radiologic studies; patients are followed for a minimum of 10 months. Among 76 patients seen in 1973, the diagnosis of idiopathic SHL was retained in 52; more specific diagnoses were established in 24. The incidence of SHL in the general population is estimated at 10.7 cases per 100,000. Although the therapeutic protocol was individualized, all patients with idiopathic SHL were advised to restrict dietary sodium, to discontinue use of stimulants, and were given instructions regarding activity. Twenty-six patients received prednisone. Results indicate that prednisone may have a beneficial effect when administered before the 10th day of SHL. Increasing age, hypertension, diabetes, and severe vertigo were found to be poor prognostic indicators. Outcome was significantly better in patients seen early in the course of their illness than in those seen later. Possible etiologic factors include vascular dysfunction and inflammatory response. Further studies of controlled series are needed to establish standards for diagnosis of SHL and its recovery pattern.