Results on the association of dietary fiber intake with colorectal cancer risk were inconsistent among previous large prospective studies, but studies in non-Western populations are lacking. Consequently, the authors investigated the association between dietary fiber intake and the subsequent risk of colorectal cancer in a prospective cohort study of a Japanese population. Dietary fiber intake was estimated from food-frequency questionnaires comprising 44 or 52 items at the baseline survey and 138 food items at a 5-year follow-up survey. The authors identified 907 cases of colorectal adenocarcinoma diagnosed pathologically in 86,412 subjects during a 10-year follow-up from the baseline survey. After the 5-year follow-up survey, 522 cases were identified in 78,326 subjects during a 5.8-year follow-up. The authors estimated the multivariate-adjusted hazard ratios of colorectal cancer for dietary fiber intake using Cox's proportional hazards model. The authors found no statistically significant association between dietary fiber intake and colorectal cancer in analyses using data either from the baseline survey or from the 5-year follow-up survey. However, the risk of only the lowest quintile was high, compared with the second to the fifth quintiles. Furthermore, the lowest subtertile in the lowest quintile was associated with an increased risk in women (hazard ratio(Sub1 vs. Q5), 2.3; 95% confidence interval, 1.0-5.2). In conclusion, although a minor effect cannot be ruled out, the results did not support a hypothesis of a dose-dependent protective effect of dietary fiber intake against colorectal cancer.