To investigate the fine structural features of nuclei and cytoplasm in osteoclasts from patients with Paget's disease of bone, eight bone biopsies were examined. Unusual paracrystalline filamentous formations were found within the nuclei of both actively resorbing and degenerating osteoclasts. These formations were made up of parallel arranged filaments which, in most cases, clustered together regularly. The filaments were slightly argyrophilic and this helped to differentiate them from the surrounding nuclear matrix, even when they were not aggregated in clusters. Argyrophilic filaments were also found loosely aggregated inside the cytoplasm of osteoclasts. In three biopsies two additional and unusual cytoplasmic inclusions were found. The first consisted of irregularly branching, highly argyrophilic membranous strands which were located inside membrane‐bound vacuoles. The second consisted of spindle‐shaped structures enclosed by two membranes and containing three or more tubules. It is suggested that these results stand as further morphological evidence in favour of the view that a virus, possibly of the paramyxovirus class, has a pathogenetic role in Paget's disease of b