The effect of intragastric gavage administration of docosahexaenoic acid (DHA) on azoxymethane (AOM)-induced colon carcinogenesis in rats was studied. Male F344 rats were treated s.c. with 15 mg/kg of AOM once a week for 2 weeks and were given either 0.7 ml of DHA or water intragastrically twice a week starting the day before the first carcinogen treatment. The number of crypt multiplicity (number of crypts/focus) of aberrant crypt foci (ACF) in the colon were measured after 4, 12, and 36 weeks. The numbers and average crypt multiplicities of ACF induced by AOM were significantly lower after 12 and 36 weeks in animals given DHA. DHA also reduced the incidence of spontaneous ACF in animals without carcinogen treatment. Colorectal tumor incidence and number of tumors per rat after 36 weeks were slightly, but not significantly, lower in the DHA-treated group. These results suggest that DHA slightly suppresses colon carcinogenesis, and a possibility that warrants further study.