The work of Japanese research committees on the relation of viral infection to sudden deafness and acute profound deafness is briefly introduced. The criteria for making a diagnosis of sudden deafness and mumps deafness are described. A new method of detecting IgM antibody to mumps virus was developed in research done for one of the committees. Use of this method revealed that sudden deafness in 1 out of 39 patients was due to mumps virus. Experimental viral labyrinthitis was produced in guinea pigs using herpes simplex virus (types 1 and 2) and guinea pig cytomegalovirus (GPCMV). Histopathological studies were conducted, and the results are discussed and compared with those for human viral labyrinthitis. Anomaly of the cochlea was found in the offspring of a guinea pig which became pregnant just after GPCMV was inoculated into its cochlea.