Some cases of mouth cancer could be caused by a virus contracted during oral sex, scientists have warned.
Scieintists looked at tissue samples from patients
Writing in New Scientist magazine, US researchers said the human papilloma virus, which causes most cervical cancers, may also cause oral tumours.
Experts say heavy smoking or drinking causes most mouth cancers, but the HPV link could help explain why some young adults develop the rare disease.
But they stressed people did not need to alter their behaviour.
It is estimated that up to a fifth of women aged 18 to 22 in the UK carry a variety of HPV.
Scientists are working on a vaccine that would be effective against the most common strains in an attempt to cut the number of cases of cervical cancer.
They now hope a vaccine could have the added benefit of reducing oral cancers, which affect around one in 10,000 people.
In the Johns Hopkins study, also published in the Journal of the National Cancer Institute, researchers compared 1,670 patients who had oral cancers to 1,732 healthy people from Europe, Canada, Australia, Cuba, and Sudan.
They took tissue biopsies to see if the HPV virus was present.
It was found in a small number of people with oral cancer.
It was more common amongst people who reported having more than one sexual partner or who practised oral sex, than in cancer patients who smoked or chewed tobacco.
Most of those who carried the virus had the HPV16 strain - which is also the most common strain in people with cervical cancer.
The virus was equally likely to be present in men and women.
Patients with mouth cancer were also three times as likely to have antibodies against HPV, which shows they had been exposed to the virus, than the healthy people studied.
The link was even stronger in people with tumours at the back of their mouths.
Dr Raphael Viscidi, who led the research, said: "This is a major study in terms of its size. I think this will convince people."
Dr Newell Johnson, of King's College London, added: "We have known for some time that there is a small but significant group of people with oral cancer whose disease cannot be blamed on smoking and drinking because they are too young.
"In this group there must be another factor, and HPV and oral sex seems to be one likely explanation.
"This study provides the strongest evidence yet that this is the case."
But Dr Anne Szarewski of Cancer Research UK told BBC News Online people should not panic about the findings.
"It is important to add to our knowledge about cancer causes, but I wouldn't want to give the impression this is a major issue.
"Oral cancer is rare, and tobacco and alcohol are by far the biggest causes."
"Work on a vaccine against cervical cancer could also have the pleasant benefit of eliminating some oral cancers too."