Diagnostic criteria of syphilis and some other diseases are proposed from a study of 424 crania and calvariae and 250 long bones in 22 medical museums in Europe. Yaws bone lesions in Uganda and changes in Australian aboriginal bones also contributed to the establishment of these criteria. Any deductions about disease in the past or isolated populations must depend upon acceptable diagnostic criteria; post mortem damage must be recognised. In crania and calvariae the sequence of changes of Virchow's caries sicca, and in long bones nodes/expansions with superficial cavitation are sound indicators of syphilis, and of yaws and treponarid in relevant geographical areas. Attention is called to the cause of sequestra in European calvariae labelled syphilis, the absence of sequestra due to haematogenous pyogenic osteomyelitis in Australian and other aboriginal bones and possibly in Europe before the Middle Ages. The number of bones with diagnostic criteria needed to demonstrate the endemicity of a particular infection in a past community is discussed. There is also need for an extensive application of diagnostic criteria of syphilis to pre-Columbian or pre-European bones everywhere. The uncertain future of old dry diseased bones in medical museums, and the need for reference centres to provide sound advice and guidance in palaeopathology are stressed.