The modifying effect of dietary administration of the polyphenolic antioxidant flavonoid silymarin, isolated from milk thistle [Silybum marianum (L.) Gaertneri], on AOM‐induced colon carcinogenesis was investigated in male F344 rats. In the short‐term study, the effects of silymarin on the development of AOM‐induced colonic ACF, being putative precursor lesions for colonic adenocarcinoma, were assayed to predict the modifying effects of dietary silymarin on colon tumorigenesis. Also, the activity of detoxifying enzymes (GST and QR) in liver and colonic mucosa was determined in rats gavaged with silymarin. Subsequently, the possible inhibitory effects of dietary feeding of silymarin on AOM‐induced colon carcinogenesis were evaluated using a long‐term animal experiment. In the short‐term study, dietary administration of silymarin (100, 500 and 1,000 ppm in diet), either during or after carcinogen exposure, for 4 weeks caused significant reduction in the frequency of colonic ACF in a dose‐dependent manner. Silymarin given by gavage elevated the activity of detoxifying enzymes in both organs. In the long‐term experiment, dietary feeding of silymarin (100 and 500 ppm) during the initiation or postinitiation phase of AOM‐induced colon carcinogenesis reduced the incidence and multiplicity of colonic adenocarcinoma. The inhibition by feeding with 500 ppm silymarin was significant (p < 0.05 by initiation feeding and p < 0.01 by postinitiation feeding). Also, silymarin administration in the diet lowered the PCNA labeling index and increased the number of apoptotic cells in adenocarcinoma. β‐Glucuronidase activity, PGE₂ level and polyamine content were decreased in colonic mucosa. These results clearly indicate a chemopreventive ability of dietary silymarin against chemically induced colon tumorigenesis and will provide a scientific basis for progression to clinical trials of the chemoprevention of human colon cancer.