Throat cancers caused by a virus transmitted during oral sex have increased significantly in the United States in recent years, researchers reported on Monday.

The virus is the same one that causes many cases of cervical cancer: human papillomavirus (HPV) Type 16.

Researchers tested tumor samples from 271 patients with certain types of throat cancer diagnosed from 1984 to 2004. The virus was found in only 16 percent of the samples from the 1980s — but in 72 percent of those collected after 2000.

The researchers estimated that over all, throat cancers caused by the virus have increased to 2.6 per 100,000 people in 2004 from 0.8 cases per 100,000 people in 1988. If the trend continues, by 2020 the virus will be causing more throat cancer than cervical cancer, the study concluded.

Doctors in the United States and other countries had already noticed increases in throat cancers caused by HPV, but the extent was unclear.

“This is the first definitive evidence that these changes at the population level are indeed caused by HPV infection,” said Dr. Maura L. Gillison, the senior author of the new study and the chairwoman of cancer research at Ohio State University.

The research is published in The Journal of Clinical Oncology.

Dr. Kevin J. Cullen, director of the Greenebaum Cancer Center at the University of Maryland, said the study was well done. “It’s very clear that this is becoming a major epidemic,” he added.

He said his own research team had found similar increases in throat cancers in Baltimore during the last 30 years. Researchers think the trend may be attributable to increases in oral sex, particularly among younger people who think it is safer than intercourse.

Dr. Gillison said the increase in throat cancers was not a cause for panic because they still are not common. There are fewer than 10,000 cases a year. Most people with HPV do not develop cancer.

The throat tumors it causes, called oropharyngeal cancers, occur in specific areas: the base of the tongue and the area just behind the mouth, including the soft palate, tonsils, and the side and back walls of the throat. Some oropharyngeal cancers are not caused by the virus, but by tobacco and alcohol; their rates have been decreasing as smoking has declined.

Throat cancers caused by HPV are more treatable than those not caused by the virus. Median survival in throat cancer patients with the virus is 131 months; without it, 20 months. Virus-related throat cancers are far more common in men than in women, a difference that has not been explained.

HPV is commonly found in the genital region and is transmitted by intimate contact. Usually the immune system fights off the infection, but in some people the virus persists and causes cancer.

There is no screening test for oropharyngeal cancers, no equivalent of the Pap test used to find precancerous growths on the cervix, Dr. Gillison said. Oropharyngeal cancers generally are not found until symptoms start: lumpy, swollen lymph nodes in the neck, or a lingering sore throat or earache.

Patients may need surgery, radiation or chemotherapy, or some combination of those treatments.

“The side effects can be horrendous,” Dr. Gillison said. “It’s a very difficult therapy to get through.”

Two vaccines, Gardasil and Cervarix, protect against HPV Type 16 and other strains of the virus that cause cancers and genital warts. But medical groups now recommend them only for girls, to protect against cervical cancer.

Researchers think the vaccines might also prevent some throat cancers, but cannot be sure unless the vaccines are specifically tested for that purpose. The manufacturers — Merck and GlaxoSmithKline — say they have no plans to study throat cancers. But Dr. Cullen said that the vaccines were likely to prevent the throat cancers, and should be given to boys too. He said he had no connection to vaccine companies.

Dr. Gillison said the vaccine companies had paid for her research in the past, but had no role in this study, which was paid for by the National Cancer Institute, Ohio State University and the Oral Cancer Foundation.