Abstract We studied the association between dietary folate and specific K‐ras mutations in colon and rectal cancer in The Netherlands Cohort Study on diet and cancer. After 7.3 years of follow‐up, 448 colon and 160 rectal cancer patients and 3,048 sub‐cohort members (55–69 years at baseline) were available for data analyses. Mutation analysis of the K‐ras gene was carried out on all archival adenocarcinoma specimens. Case–cohort analyses were used to compute adjusted incidence rate ratios (RR) and 95% confidence intervals (CI) for colon and rectal cancer overall and for K‐ras mutation status subgroups according to 100 μg/day increased intake in dietary folate. Dietary folate intake was not significantly associated with colon cancer risk for men or women, neither overall nor with K‐ras mutation status. For rectal cancer, folate intake was associated with a decreased disease risk in men and was most pronounced for K‐ras mutated tumors, whereas an increased association was observed for women. Regarding the K‐ras mutation status in women, an increased association was observed for both wild‐type and mutated K‐ras tumors. Specifically, folate intake was associated with an increased risk of G>T and G>C transversions in rectal tumors (RR = 2.69, 95% CI = 1.43–5.09), but inversely associated with G>A transitions (RR = 0.08, 95% CI = 0.01–0.53). Our data suggest that the effect of folate on rectal cancer risk is different for men and women and depends on the K‐ras mutation status of the tumor. © 2004 Wiley‐Liss, Inc.