Paget's disease of bone (PDB) is a chronic disorder characterized by focal abnormalities of bone turnover. The symptoms of PDB include bone pain, bone deformity, osteoarthritis, and an increase in risk of pathological fracture and neurological problems. PDB is the second most common metabolic bone disease in European countries, although PDB is uncommon in Japan. PDB shows geographical and ethnic clustering. Based on this, two hypotheses of etiology of PDB have been proposed. One hypothesis is that PDB results from a slow virus infection of osteoclasts with paramyxovirus. The other is that PDB is a genetic disease. The development of bisphosphonate has brought major changes to the treatment of PDB. In Japan, etidronate and calcitonin are approved by the Ministry of Health, Labour and Welfare for treating PDB. Risedronate is under development for treating PDB in Japan.