Even after adjusting for the other risk factors, the new study results support the evidence that people who smoke cigarettes over a long period of time have an increased risk for developing colo-rectal cancer, even after adjusting for other risk factors.

Senior author Michael J. Thun, M. D., M. S., vice president emeritus, epidemiology and surveillance research at the American Cancer Society said, "This provides one more reason not to smoke, or to quit as soon as possible, colorectal cancer should be added to the list of cancers caused by smoking."

Findings are published in the December issue of Cancer Epidemiology, Biomarkers & Preventio, a journal of the American Association for Cancer Research, as part of a special focus on tobacco.

Those who were smoking for 40 years or more, or who did not quit before age 40 had a 30 percent to a 50 percent increased risk of developing colon or rectal cancer during the follow-up, even in analyses that adjusted for 13 other potential risk factors as per Thun.