A survey of the literature concerning sudden deafness caused by virus infection is followed by a discussion of 46 patients with the history of acute loss of hearing, who were subjected to virological-serological examination. In 9 patients the titre movement of the complement fixation reaction was regarded to imply a viral cause of the acute loss of hearing. This was corroborated in 5 cases by the history of recent viral infection. Pathogenetically, the significance of the viral endolymphatic labyrinthitis was underlined. The success of traditional therapy did not differ from the results in cases of sudden deafness due to factors other than viral disease. In the absence of specific therapy the importance of prophylactic measures is stressed.