The researchers found colorectal cancer death rates were lowest among people who had never smoked, intermediate among ex-smokers and highest among current smokers. In one year ? 1997 ? smoking may have been responsible for about 12% of deaths from colorectal cancer, accounting for more than 6,800 deaths from the disease that year, according to the researchers.

The ACS scientists studied data on 312,332 men and 469,019 women collected as part of the Society?s Cancer Prevention Study II (CPS II). They examined smoking patterns reported by participants when they entered the study in 1982, including if they smoked and for how long. The scientists then looked at the causes of death among study participants who died between 1982 and 1997. Along with the smoking risk factor, they also took into account any increases in colorectal cancer risk caused by other factors, such as eating a high-fat diet, with too few fruits and vegetables, getting too little exercise and being overweight.

The researchers found risk of dying from colorectal cancer increased as the number of cigarettes smoked daily and the number of years of smoking went up and as the age at which people began smoking went down. The good news is that colorectal cancer risk decreased with each year after quitting smoking. "This shows another reason it?s best not to start smoking at all," says Ann Chao, PhD, a research epidemiologist with the ACS and lead author of the study. "However, quitting early lowers the colorectal cancer death rate."

Another finding was that women who smoked were more than 40% more likely to die from colorectal cancer than women who never had smoked. Male smokers had more than a 30% increase in risk of dying from the disease compared to men who never had smoked.

"The smoking epidemic in women began years later than in men, and this may explain in part why colorectal cancer death rates were higher in men than in women during the 50s, 60s and 70s," says Michael Thun, MD, vice president of epidemiology and surveillance research for the ACS. "As the smoking rates increased for women, the colorectal cancer death rates became similar."

Cigarette smoking has previously been shown to cause cancer of the lung, mouth, pharnyx, larynx, esophagus, pancreas, kidney and bladder. Cancer-causing chemicals present in cigarette smoke come in contact with tissues in the body either directly, when inhaled smoke touches them, or later, when the bloodstream carries the chemicals to tissues.

In addition to providing another reason not to smoke, the researchers say this link between smoking and colorectal cancer means that getting early detection tests is especially important for current and former smokers

And Chao says the large numbers of people studied makes scientists more certain now of what had earlier been suspected: that smoking may cause a very significant portion of colorectal cancers.

"Findings from the American Cancer Society study and other large studies suggest that colorectal cancer should be reconsidered for classification as a smoking-related cancer," she says.