The etiology of Paget's disease of bone has long remained obscure. The recent discovery in electron microscopy of specific inclusions in the cytoplasm and nuclei of osteoclasts in tissue from patients with Paget's bone disease has been the starting point of investigations into a possible viral origin. The inclusions, made up of microcylinders, described by several authors as being found only in osteoclasts in Paget's bone disease, present a close morphological analogy with nucleocapsids of paramyxovirus of the measles group. Immunocytological methods have provided a complementary approach to the problem. It has been demonstrated that the osteoclasts in Paget's bone disease contain antigenic material which reacts positively with sera containing measles antibodies. Both the morphological and the immunocytological evidence is strongly in favour of a viral etiology for Paget's bone disease.