Ultrastructural studies of the bone cells in patients with Paget's disease of bone have revealed the presence of nuclear and cytoplasmic inclusions that resemble the nucleocapsids of Paramyxoviridae virus. Immunocytologic studies of bone biopsy specimens and cultured bone cells have demonstrated positive responses with antisera against measles virus and/or respiratory syncytial virus. These observations suggest that Paget's disease may be a slow viral infection of bone. The exact nature of the putative viral agent has yet to be established.