Although tobacco has been clearly implicated as a cause of a number of cancers, earlier studies had not generally found cigarette smokers to be at higher risk for cancers of the large bowel. Unlike the earlier studies, more recent studies have tended to find cigarette smokers to be at higher risk of colorectal cancer. In addition, cigarette smoking has been consistently associated with a higher risk of colorectal adenomas, precursors of cancers. We hypothesize that smoking is an initiator of colorectal carcinogenesis. The consistent finding of a positive association between smoking and risk of adenomas may reflect the presumably shorter induction period for these lesions. Recent studies indicate that an increased risk for large-bowel cancer emerges only after about four decades after one begins smoking, perhaps accounting for the null studies that were conducted earlier during the smoking epidemic. Most studies had not taken into account an induction period between timing of smoking and risk for cancer. The increasing male-to-female mortality ratio from colorectal cancer over the latter half of this century in the United States may have been a result of tobacco use by men earlier in the century. If the associations observed with colorectal adenomas and cancers in recent studies are causal, approximately 20% of the large-bowel cancers in men would be attributable to smoking. On the basis of the strength of the available evidence, intensified effort to prevent smoking among young people is warranted.