If any waste remains, flat lesions will be buried by it. Studies have shown that in about a quarter of all colonoscopies, the bowel preparation is inadequate.

Dr. Rex said that male veterans tended to have more precancerous colon growths than other groups, so the rate of flat lesions in women or the general population might not be quite as high as those in the study.

Dr. Soetikno and his colleagues started an exchange program with doctors in Japan to learn their techniques for recognizing and removing the flat lesions.

American doctors should learn from overseas colleagues more often, Dr. Rothenberger said, adding, “We tend to get very smug about our abilities.”

The quality of colonoscopy has become a delicate issue, because an article in The New England Journal of Medicine in December 2006 found that some doctors were 10 times better than others at finding precancerous polyps. A major factor in their success was taking enough time to examine the colon thoroughly, as opposed to rushing through the procedure. Doctors who miss polyps would almost certainly miss flat lesions as well because they are harder to see. The new study underscores the need for careful examinations, because the flat lesions are more dangerous.

The study also raises doubts about whether “virtual colonoscopy,” performed by a CT scanner, will ever be able to take the place of the colonoscope inserted into the rectum, as many patients had hoped. The problem is that CT scans use X-rays to reveal shapes, and find polyps because they stick out. Flat lesions are unlikely to show up in such scans.

Studies show that from 0.3 percent to 0.9 percent of patients develop colon cancers within just a few years of having a colonoscopy and polyp removal  exactly what the procedure is supposed to prevent. Some doctors think that flat lesions, missed entirely during the colonoscopy or not fully removed, may account for some of these apparent failures.