Chester's Georgian residents were eight times more likely to have syphilis than their rural neighbours, a study of sex and disease in the period has found.
About 8% of the city's under-35s were infected, compared to 1% in the countryside, according to the study's author Prof Simon Szreter.
It was no "hotbed of prostitution" though and equal numbers of men and women suffered from "the pox", he said.
However, he added that city dwellers did have more "non-marital sex".
Prof Szreter, of St John's College, University of Cambridge, estimated the figures using Chester Infirmary's admissions register from 1773 to 1775 and a 1774 census by local physician John Haygarth - which pre-dates the national census that began in 1801.
He identified possible syphilitic cases based on the length of stay among 177 cases of "venereal distemper", and compared estimates concerning the urban population with those for rural residents living within a 10-mile radius.
He said the figures "suggest there's less opportunity for non-marital sex in the countryside, or more opportunity in the city, but it would be speculative to go further than that".
Syphilis in 18th Century
- The treatment process for cases thought to be syphilis, known as "the pox", took at least 35 days
- It involved the continual application of mercury, which caused patients to produce pints of saliva, supposedly flushing out the venereal poisons
- Side-effects included swollen gums, mouth ulcers and severe bad breath
Source: Continuity and Change journal
There was no evidence that people in rural communities were less likely to seek medical help, he added.
"If you've a society where everyone is monogamous and faithful, you have no STIs (sexually transmitted infections)."
Data about STIs prior to the 20th century is not widely available, and the rate has therefore been seen as incalculable.
"It is the first time that we have had any historical statistical evidence like this for sexual disease rates anywhere in Britain," Prof Szreter said.
"The demographic story of this period is defined by mortality and fertility, and rates of venereal disease could, of course, affect both.
"But because we haven't been able to study the impact of STIs, much of the history of British population change has been written as if there wasn't any."
The study, Treatment Rates for the Pox in early modern England, has been published in the journal Continuity and Change.