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OBJECTIVE: Fish oil supplementation may represent a potential chemopreventive agent for reducing colorectal cancer risk. Fish oil’s mechanism of action is unknown but presumed to be related to eicosanoid modification. The purpose of this study was to evaluate the effects of fish oil supplementation on levels of urinary and rectal eicosanoids. METHODS: We conducted a randomized, double-blind, controlled trial of 2.5 grams of fish oil per day compared to olive oil supplementation over a six month period. Study participants had a history of colorectal adenomas. Randomization was stratified based on the gene variant rs174535 in the fatty acid desaturase 1 enzyme (FADS1), which affects tissue levels of arachidonic acid. RESULTS: A total of 141 subjects were randomized. Urinary prostaglandin E(2) metabolite (PGE-M) was measured at baseline, 3 months, and 6 months and rectal prostaglandin E(2) (PGE(2)) at baseline and 6 months. Repeated measures linear regression was used to determine the effect of the intervention on each outcome measure. Overall, fish oil supplementation was found to reduce urinary PGE-M production compared to olive oil (P = 0.03). Fish oil did not reduce rectal PGE(2) overall, however it did significantly reduce PGE(2) in the subgroup of participants not using aspirin or NSAIDs (P=0.04). FADS1 genotype did not seem to modify fish oils effects on PGE(2) production. CONCLUSIONS: We conclude that fish oil supplementation has a modest but beneficial effect on eicosanoids associated with colorectal carcinogenesis, particularly in those not taking aspirin or NSAIDs.