jats:titleABSTRACT</jats:title>jats:pThis article offers an innovative attempt to construct an empirically-based estimate of the extent of syphilis prevailing in two contrasting populations in late eighteenth-century Britain. Thanks to the co-incident survival of both a detailed admissions register for Chester Infirmary and a pioneering census of the city of Chester in 1774 taken by Dr John Haygarth, it is possible to produce age-specific estimates of the extent to which adults of each sex had been treated for the pox by age 35. These estimates can be produced both for the resident population of Chester city and comparatively for the rural region immediately surrounding Chester. These are the first estimates of the prevalence of this important disease produced for the eighteenth century and they can be compared with similar figures for England and Wales c. 1911–1912.</jats:p>