Paget's disease of bone (PDB) manifests fairly late in life and currently affects up to 3% of individuals older than 55 years of age. An about 60% decline in the prevalence of PDB between 1970 and 1990 was documented in England, particularly in Lancashire, where the disease was initially most common. Although wide regional variations exist, the overall prevalence is decreasing, as confirmed by a recent meta-analysis. This secular decline is associated with a decrease in mortality due to the disappearance of sarcomatous transformation of PDB lesions. Another reported change is a decrease in the severity of the clinical PDB phenotype, with a gradual increase in the age at diagnosis of 4 years per decade. In familial forms related to an SQSTM1 mutation, the first manifestations are detected about 10 years later from one generation to the next, and the disease is less severe, with smaller elevations in serum alkaline phosphatase levels and less extensive lesions. Although incompletely understood, the reasons for these changes might involve environmental factors such as the correction of childhood deficiencies in calcium and/or vitamin D, a less rural lifestyle, and decreased contact with domestic animals and consumption of bovine organ meats during childhood. Childhood exposure to industrial waste and products of combustion has also been incriminated. Finally, although classical, the role for paramyxoviruses such as the measles virus in the pathogenesis of PDB remains debated in the literature.