During the last year patients presenting with progressive bilateral or sudden sensorineural deafness of unknown aetiology have been investigated for possible abnormal immune activity. Twenty-six cases are reported in which significantly raised circulating immune complexes were present, together with 2 cases of clinical autoimmune deafness where the complexes were normal. Thirty-six control subjects were also studied. Following a review of the clinical features, the pathogenesis of this new association with sensorineural deafness is discussed. In some cases the aetiology is probably autoimmune, and in others related to infection. In certain patients the deafness has been partly reversed by medical treatment with systemic steroids or plasma exchange. It is hypothesized that circulating immune complexes may reflect a previously unrecognized final common pathophysiological pathway in a variety of cochleovestibular disorders.