Paget disease of bone was first described more than 100 years ago, but its etiology has not been clearly defined. Ultrastructural and immunocytochemical studies have suggested a viral etiology for Paget disease. Farther support for this hypothesis has been provided by in situ hybridization studies, which have shown measles virus nucleocapsid transcripts and canine distemper virus transcripts in pagetic bone cells. Recently, studies using the polymerase chain reaction demonstrated measles virus nucleocapsid messenger RNA and canine distemper virus messenger RNA in samples of RNA isolated from pagetic bone However, other workers have been unable to detect paramyxoviral sequences in RNA similarly isolated. In addition, several problems exist with the viral hypothesis. Paget patients do not have elevated liters of paramyxovirus antibodies in their sera, nor does the geographic distribution of paramyxoviral infections correspond to that of Paget disease. Until viral products are cloned, completely sequenced, and transfected in normal osteoclasts, and these osteoclasts are made to express a pagetic phenotype, the viral etiology of Paget disease will remain a hypothesis.