Tobacco smoking was studied in relation to colorectal cancer in 56 973 Finnish men and women initially free from cancer. Smoking status was determined by a health questionnaire. During a follow-up period of 28 years, from the baseline in 1966-72 to the end of 1994, 457 cases of colorectal cancer occurred. There was no significant association between baseline smoking status and colorectal cancer risk over the total follow-up period. The sex- and age-adjusted relative risk of colorectal cancer between smokers and non-smokers was 1.06 (95% confidence interval 0.84-1.33). For follow-up periods of 11-20 years, however, the relative risk was 1.57 (95% confidence interval 1.09-2.24). In a subgroup in which smoking habits were assessed twice, the relative risk of colorectal cancer among persistent smokers was 1.71 (95% confidence interval 1.09-2.68) compared with others. The results of the present prospective study are consistent with the possibility that smoking increases the risk of colorectal cancer after a relatively long induction period. To clarify the role of smoking in colorectal cancer development, further cohort studies are needed with long follow-up periods and allowing for control of dietary and other potential confounding factors.