Background: Colorectal cancer is a major public health problem in both North America and western Europe, and incidence and mortality rates are rapidly increasing in many previously low-risk countries. It has been hypothesized that increased intakes of fiber, vitamin C, and beta carotene could decrease the risk of colorectal cancer. Purpose : The objective of this study was to examine the effects of fiber, vitamin C, and beta-carotene intakes on colorectal cancer risk in a combined analysis of data from 13 case-control studies previously conducted in populations with differing colorectal cancer rates and dietary practices. The study was designed to estimate risks in the pooled data, to test the consistency of the associations across the studies, and to examine interactions of the effects of the nutrients with cancer site, sex, and age. Methods : Original data records for 5287 case subjects with colorectal cancer and 10470 control subjects without disease were combined. Logistic regression analysis was used to estimate relative risks and confidence intervals for intakes of fiber, vitamin C, and beta carotene, with the effects of study, sex, and age group being adjusted by stratification. Results : Risk decreased as fiber intake increased; relative risks were 0.79, 0.69, 0.63, and 0.53 for the four highest quintiles of intake compared with the lowest quintile (trend, P <.0001). The inverse association with fiber is seen in 12 of the 13 studies and is similar in magnitude for left- and right-sided colon and rectal cancers, for men and for women, and for different age groups. In contrast, after adjustment for fiber intake, only weak inverse associations are seen for the intakes of vitamin C and beta carotene. Conclusion : This analysis provides substantive evidence that intake of fiber-rich foods is inversely related to risk of cancers of both the colon and rectum. Implications : If causality is assumed, we estimate that risk of colorectal cancer in the U.S. population could be reduced about 31% (50000 cases annually) by an average increase in fiber intake from food sources of about 13 g/d, corresponding to an average increase of about 70%. [J Natl Cancer Inst 84:1887–1896, 1992]