By use of the very sensitive polymerase chain reaction technique, evidence is given that otosclerosis is a measles-virus-associated disease of the otic capsule. These results support recent observations which demonstrated the expression of measles antigen within active otosclerotic foci by immunohistochemical methods. Thus it is hypothesized that the viral infection acts as at least one pathogenetic factor during the development of this obscure, locally restricted inflammatory bone disease.