Smoking habits were ascertained by interview from Wisconsin women aged 30-74 years with newly reported diagnoses of colon (n = 536) and rectal (n = 243) cancer and 2315 randomly selected population controls. After controlling for age, body mass index, alcohol consumption, family history of large bowel cancer, and history of screening sigmoidoscopy, significantly elevated risks were observed for women who had ever smoked, in both the colon (odds ratio, 1.28; 95% confidence interval, 1.03-1.58) and rectum (odds ratio, 1.44; 95% confidence interval, 1.08-1.92). Risk significantly increased with greater number of cigarettes smoked per day, longer duration of smoking, and earlier age at initiation for both the colon and the rectum; however, only duration of smoking was not independently associated with risk. Among former smokers, risk for both colon and rectal cancer remained elevated. These data suggest that women who smoke are at elevated risk of both colon and rectal cancer and that increased risk persists even among former smokers.