Many people maintain rich, active sex lives well into their 80s, according to the first detailed examination of sexuality among older Americans.

The nationally representative survey of more than 3,000 American adults ages 57 to 85 found that more than half to three-quarters of those questioned remain sexually active, with a significant proportion engaging in frequent and varied sexual behavior.

Sexual problems do increase with age, and the rate of sexual activity fades somewhat, the survey found. But interest in sex remains high and the frequency remains surprisingly stable among the physically able who are lucky enough to still have partners.

“There’s a popular perception that older people aren’t as interested in sex as younger people,” said Stacy Tessler Lindau of the University of Chicago, who led the study being published today in the New England Journal of Medicine. “Our study shows that’s simply not true. Older people value sexuality as an important part of life.”

“This study paints a portrait of this aspect of older Americans’ lives that suggests a previously uncharacterized vitality and interest in sexuality , ” said Georgeanne Patmios of the National Institute on Aging, the primary funder of the study. “This has not perhaps been fully appreciated.”

The survey found a close link between sex and health, with healthier people reporting the highest rates of sexual activity. In addition to supporting the well-known idea that illness can interfere with sex, that finding suggests that a healthy sex life may itself help keep people vibrant, the researchers said.

“Individuals who remain sexually active gain the benefit of the physical exercise that comes with sex,” Ms. Lindau said. “It’s also possible the hormones — the endorphins released by orgasms — give a general sense of well-being that could be beneficial. The psychological benefits of being loved and cared for may also trickle over to physical health.”

Despite the intense focus on sex in popular culture, political sensitivities have severely limited funding for reliable, detailed studies of sexual activity among Americans of any age. Smaller, more limited studies have provided glimpses into the sex lives of the elderly, but no one had previously attempted an in-depth, nationally representative survey among this rapidly growing segment of the population.

“We just don’t know very much about sexuality in the later years,” said Robert Butler, president of the International Longevity Center in New York, a nonprofit think tank. “There’s been a tremendous amount of resistance to such studies. That’s what makes this so terrific.”

For the study, researchers conducted face-to-face interviews with a randomly selected sample of 3,005 Americans between July 2005 and March 2006.

“We found people to be grateful to have an opportunity to discuss these issues,” said Ms. Lindau, noting that researchers achieved an unusually high 75% response rate from those they approached. “The topics we were asking about resonated with people. Many said they had never had a chance to talk to anyone about these issues, not even a spouse or their physicians.”

About 28% of men and about 14% of women said sex was very important, and about three-quarters of those with partners reported being sexually active, which is about equivalent to what previous research had found for people in their 40s and 50s. Being sexually active was defined as having had mutually voluntary sexual contact with another person within the past 12 months.

“Our findings indicate that when it comes to sexual activity, older people are really just younger people later in life,” Ms. Lindau said “There’s no reason to believe they give up the basic human desire for love and intimacy, and the kind of pleasure that comes from intimate relationships.”

The proportion of those having sex did decline somewhat with age. By ages 75 to 85, it was down to 39% of men and 17% of women. Among those who remained sexually active, frequency also fell somewhat with age. But even among the oldest age group, 54% of those who were sexually active reported having sex at least two to three times a month and 23% reported having sex once a week or more.