Two of four patients with active stapedial otospongiosis had abnormal filamentous structures in osteoblast-like cells. These structures were rare in these cells and did not occur in any other cells. The structures were 17 nm wide and clustered in a loose skein in dilated cisternae of the rough endoplasmic reticulum. They resembled the nucleocapsids of subacute sclerosing panenecephalitis, a variant of measles virus.