The findings come from a study of colonoscopy, in which a camera-tipped tube is used to examine the lining of the intestine. Generally, doctors search for polyps, abnormal growths that stick out from the lining and can turn into cancer. But another type of growth is much more dangerous, and harder to see because it is flat or depressed and similar in color to healthy tissue.

Japanese researchers became concerned about these flat lesions in the 1980s and 1990s, but studies here had mixed results, and American doctors tended to think that flat growths were less common and less dangerous in the United States.

The new study, published recently in the Journal of the American Medical Association, suggests otherwise.

Some doctors in this country were already alert to flat lesions, but the findings will pose a challenge to others because it takes a trained and vigilant eye to see the growths and special techniques to remove them. The results also mean it is especially important that patients take the harsh laxatives that many dread in advance of the test. The flat lesions, hard to find even under the best conditions, will be impossible to see if any waste is left in the bowel.

Colon cancer is the second-leading cause of cancer death in the United States, after lung cancer, with about 154,000 new cases detected and 52,000 deaths a year. It is one of the few cancers that are totally preventable if precancerous growths are found and removed; it can also be cured with surgery alone if found early enough.

People who have just had a colonoscopy should not rush to schedule another one just to look for the flat growths, doctors said.

"I don’t think people have to panic that they’ve somehow been neglected and had poor care," said Dr. David A. Rothenberger, deputy chairman of surgery at the University of Minnesota.

But he and other experts emphasized that people should see a doctor any time they have persisting symptoms that could indicate colon cancer, like rectal bleeding or a change in bowel habits -- no matter how recently they had a colonoscopy. The test is highly reliable, but not perfect, doctors say.

Some doctors who perform colonoscopy just are not good at seeing flat lesions, but may improve with training and practice, said Dr. Douglas K. Rex, a gastroenterologist and professor of medicine at Indiana University.

"I think there are people who expect everything in there to be shaped like a golf ball," he said. "It’s not."

Dr. David Lieberman, chief of gastroenterology at Oregon Health and Science University, who wrote an editorial accompanying the study, said: "I think there will be some surprise. There has been in general some skepticism in the United States about how common flat and depressed lesions are and how important they are. So I think this study, coming from the United States and from a good group of investigators, will be a wake-up call to a lot of physicians and will prompt people to be looking for these lesions."

The study, of 1,819 military veterans, mostly men, found that 9.35 percent had flat lesions, and those lesions were five times as likely as polyps to contain cancerous or precancerous tissue. Depressed or indented lesions were the least common but the most risky. Together, the flat or depressed lesions accounted for only 15 percent of the potentially cancerous growths found in the study, but were involved in half of the cancers. Once the doctors spotted the flat lesions, they sprayed a bluish dye on them to see their outlines better and remove them completely.