In spite of the large number of theories advanced to clarify the etiology of Paget's disease, its cause is still being discussed, and no satisfactory conclusion has been reached. The possibility of a viral origin was raised by the discovery of inclusion bodies, detectable by electron microscopy, in the nuclei and cytoplasm of the osteoclasts in the affected bone tissue, in 1974. In fact, the microcylindrical structures described by various authors, and visible only in osteoclasts and only in Paget's disease, if one excepts certain giant-cell bone tumors, have a close analogy, morphologically, with the nucleocapsids of paramyxovirus of the measles group, described in experimental infections or human diseases (subacute sclerosing panencephalitis). Various morphological arguments, drawn from studies of inclusions in richly nucleated giant osteoclasts found in Paget's disease, are in favor of the viral nature of these formations. Immunocytological methods have constituted another approach to the problem raised by the discovery of inclusions. They have demonstrated the existence of an antigenic material in the osteoclasts found in Paget's disease which reacts positively with antiserums containing anti-measles antibodies or with produce a crossed reaction with them. Controlled tests have confirmed these findings. Biological arguments are presently sufficient, therefore, for the possibility of a viral etiology of Paget's disease to be validly accepted from among the pathogenic hypotheses proposed for a disease that was first described a century ago.