Paget's disease of bone (PDB) is reportedly declining in prevalence and severity, with increasing numbers of monostotic cases. Some accounts suggest that these findings are more evident in women, and that monostotic disease is unexpectedly frequent at certain sites. We have studied whether birth date or gender is associated with the number of sites affected and with the distribution of sites in monostotic disease and, by reviewing 100 follow-up 99Tc(m) methylene diphosphonate (MDP) scans, whether additional sites appear after initial diagnosis. Scintigraphic scans from 171 male (age 40-91 years) and 179 female (44-88 years) consecutive referrals with PDB were reviewed. Patients were analysed by referral date (1982-1992 and 1993-2001), and by their median date of birth (before (PRE21) and after (POST21) 1921). Mean age of pre-1993 patients was 69 years and 75 years for referrals after 1993. Younger patients had more monostotic disease (POST21 vs PRE21 subjects (47% vs 28%)), with a more marked trend in females (52% vs 25%), and POST21 females had fewer polyostotic sites than males (p<0.05), whereas the number in PRE21 males and females was similar. Monostotic females, but not males, showed an excess of tibial involvement. The spine was less involved in monostotic disease. Follow-up scans in 100 patients revealed no new sites. The incidence of monostotic disease has doubled over the last 30 years, but diminishing site involvement appears to be more marked in females. The lesser involvement at the axial sites in monostotic disease may lead to overestimation of the decline in PDB based on abdominal radiographs.