In July 2005, Jeff Carenza and his girlfriend were enjoying a getaway weekend in Miami when food poisoning landed them both in the hospital. Blood tests showed that Dr. Carenza, then 29, had iron-deficiency anemia.

''It''s probably nothing," the doctor told him. "But have it checked when you get home."

This type of anemia can be caused by blood loss from the intestinal tract. So back in St. Louis, where Dr. Carenza was a radiology resident at Barnes-Jewish Hospital, his doctor sent him to a gastroenterologist.

One possibility was that he had a growth in his colon. But the gastroenterologist was dismissive. ''''This is going to be a waste of my time,'''' he said before performing a colonoscopy.

Half an hour later, Dr. Carenza received bad news.

"There''s a large tumor in your transverse colon," the gastroenterologist said. "It''s so big I couldn''t even get my scope around it. It''s probably been there for some time."

Dr. Carenza recalled, "My stomach and heart were on the floor."

The diagnosis, he added, "went against what we''re trained to think in medical school" - that "cancer happens to older people."

Cancer is indeed less common in adolescents and young adults than in older people. But in some ways it is more alarming.

The rates of some cancers, including colorectal, thyroid and testicular, are rising in people ages 20 to 39, according to the American Cancer Society. And a report released in August by the National Cancer Institute and Lance Armstrong Foundation found that even as cancer survival rates continued to improve in adults of middle age and older, the survival rates for people ages 15 to 39 had not risen substantially in more than two decades.

Cancer is the fourth-leading killer in that age group, behind accidents, suicides and homicides. In 2006, there were approximately 55,200 new cases of cancer in Americans 15 to 39, and 9,300 cancer deaths in that same group, according to the American Cancer Society.

"Cancer in adolescents and young adults presents distinctive issues," said an author of that report, Dr. Karen Albritton, director of adolescent and young adult oncology at the Dana-Farber Cancer Institute in Boston. "The gaps in our knowledge are huge."

Some experts say more information is needed before devoting extra resources to adolescents and young adults. Dr. Bruce A. Chabner, clinical director of the Massachusetts General Hospital Cancer Center, notes that overall survival rates of young adults are still better than those of older adults, reflecting the differences in tumor types and better tolerance of treatment in younger patients.

As of 2003, the last year for which statistics were available, the five-year survival rate was 78.5 percent for people ages 20 to 39, and 68 percent for those 40 to 69, according to the American Cancer Society.

But Dr. Albritton and others say research is needed to learn why some young adults are predisposed to cancer, why the rates of some cancers are rising in this age group and how young people may respond differently to treatment regimens - many of which have been tested mostly in older adults or in children.

Young people''s tumors may have distinct genetic and biological features, these researchers say. And survivors face unique psychosocial issues - as they think about future fertility, finishing school or starting a career.

Dr. Carenza said that looking back, he should have paid more attention to warning signs. An avid runner, he was accustomed to jogging five or six miles several days a week but for months had been unable to do so. He had also grown noticeably pale.

Still, he recalled, even his mother brushed that aside, saying: "Well, he''s a radiologist. He works in a dark room all day."

Dr. David Dietz, the colorectal surgeon at Barnes-Jewish Hospital who treated Dr. Carenza, says cancer in younger patients is typically diagnosed at a later stage because neither they nor their doctors expect them to have it. Nor do they receive routine screening, as older people do.

Dr. Carenza has no family history of cancer, and tests after the tumor was found were negative for all known hereditary colorectal cancer syndromes. (He completed chemotherapy last March and has since been in remission, though he sees an oncologist every three months.) Still, genetic factors may be more likely to play a role when young people get the disease, Dr. Dietz said, adding, ''''That is something we would like to know more about.''''

Asha Mevlana of Los Angeles had a family history of cancer. She found a lump in her breast at 22, but her cancer was not diagnosed for more than two years.

Ms. Mevlana received a mammogram early on, but the results were inconclusive, as they sometimes are in younger women whose breast tissue is denser.

"My doctor said, ''Don''t worry about that; you''re way too young for breast cancer,''"  she said. "Which is of course what I wanted to hear, so I let it go."

But as the lump grew and began to hurt, Ms. Mevlana''s mother insisted she go to another doctor and get a biopsy, which confirmed that she had breast cancer.

"It changes your perspective on everything," she said. "You think: ''Wow, I could be dead next year. And there are so many things I haven''t done.''"

Ms. Mevlana hoped to begin a career as an electric violinist. And in considering treatment options, one priority was to avoid drugs that cause neuropathy, a numbness and tingling in the hands or feet that is a side effect of some chemotherapy drugs.

The choices were "completely confusing," she said. Several doctors suggested different regimens but told her that most of them had not been well studied in younger women.

One doctor she consulted was Nadine Tung, director of the cancer risk and prevention program at Beth Israel Deaconess Medical Center in Boston. In an interview, Dr. Tung agreed that there had been few clinical trials to assess treatment regimens specifically in young adults.

"That has been a problem," she said, adding that some important research was "finally happening."

Unlike most young breast cancer patients, Ms. Mevlana had a tumor that was stimulated by estrogen. In such cases, Dr. Tung said, an important question is whether menstruation should be artificially blocked after chemotherapy, by giving the patient a shot or by removing her ovaries.

Ms. Mevlana said one doctor advised ovary removal. But she decided against it because she did not want to face the prospect of not having children.

"It was just too much to give that up," said Ms. Mevlana, who is now 31 and is a professional violinist.

Dr. Albritton of Dana-Farber says that adolescents and young adults often respond differently to treatments, and that there is a "disparity of attention" to them in the medical community.

There need to be "more clinical trials and more providers trained specifically for young adults," she said.

Researchers also say further work is needed to understand why the rates of certain cancers are rising among young adults.

Testicular cancer, for example, is most common among young men, and its rate is increasing. "We do not know why this is the case," said Dr. Tongzhang Zheng of the Yale School of Public Health, who recently received a $5.5 million grant from the National Cancer Institute to study potential genetic and environmental risk factors for the disease.

Nor is it clear why the rate of thyroid cancer, which largely affects young people, is rising, said Dr. R. Michael Tuttle, an endocrinologist and thyroid cancer expert atMemorial Sloan-Kettering Cancer Center.

"The only thing we know of that causes thyroid cancer is radiation," Dr. Tuttle said, noting that people living downwind of Chernobyl had higher rates of this cancer. "But the spike in the U.S. is not being caused by radiation."

Dr. Archie Bleyer, a specialist in adolescent and young-adult cancer at the CureSearch National Childhood Cancer Foundation, says there is a "uniqueness" to cancer in this age group that goes beyond the scientific and clinical issues. Not only do young patients face a distinctive array of cancers and therapies, but they are also confronted with personal challenges that are different from those affecting either older adults or children.

Young people are more likely to be uninsured or underinsured, he said - one reason their diseases tend to be diagnosed later. They are also more likely to feel stigmatized, since fewer of their peers have had experience with cancer.

And they face huge developmental challenges - entering society, joining the work force, contemplating marriage and starting a family - which can be overwhelming even without the burden of disease.

"One of the biggest struggles in doing anything medically is to overcome the psychosocial challenges," Dr. Bleyer said.

A growing number of advocacy and outreach groups have been founded for young adults - many of them by cancer survivors. The Colon Club, for instance, aims to raise awareness about colon cancer in young people through projects like creating a yearly calendar of survivors. The Young Survival Coalition focuses on awareness and advocacy regarding breast cancer in young adults. And the SAMFund provides financial support to young survivors making the transition to post-treatment life.

More than 50 organizations dedicated to cancer-related issues for young adults are cataloged on a new Web site, imtooyoungforthis.org, started by Steps for Living, a nonprofit communications, advocacy and support organization for young adults with cancer.

"When I was diagnosed with cancer, all I really wanted was to connect to other people my age," said the group''s founder, Matthew Zachary, 32, of Brooklyn, who learned at 21 that he had pediatric brain cancer.

Lauren Terrazzano, who writes a weekly column for Newsday called "Life, With Cancer," said that for a younger person, "cancer can be the ultimate form of identity theft, if you let it."

Ms. Terrazzano was told she had advanced lung cancer in 2004 and has since undergone three operations, including an extrapleural pneumonectomy, in which her right lung and part of the lining of her heart were removed. She has also undergone extensive chemotherapy and radiation.

"One day you''re a healthy 36-year-old with long hair, black mascara and low-rider jeans," she said. "You''re going through the rhythms of life, going to work, going to dinner, hanging out with friends. And the next minute you''re Cancer Girl."

"It''s a hard thing to reconcile."