The authors evaluated clinical outcomes during and after chemotherapy in colorectal cancer patients supplemented with fish oil during the first 9 wk of treatment. Thirty individuals never submitted to chemotherapy were randomized into supplemented group (SG), which received 2 g/day of fish oil (0.6 g/day of EPA and DHA) for 9 wk or control group (CG), which received neither fish oil nor placebo. Outcomes assessed were number of chemotherapy cycles administered; days undergoing chemotherapy; number of delays and interruptions in the administration of chemotherapy; number of hospitalizations during chemotherapy; tumor progression; values of carcinoembryonic antigen (CEA); days until events (death and progression); and 3 yr survival. Time to tumor progression was significantly longer in SG [S593 days (±211.5)] vs. CG [330 days (± 135.1); P = 0.04], other outcomes did not differ between groups. Subjects with advanced cancer who received fish oil presented longer time to tumor progression and lower CEA values after chemotherapy; however these differences were not statistically significant. Supplementation with 2 g/day of fish oil for the first 9 wk of chemotherapy may contribute to delay in tumor progression in colorectal patients, possibly by enhancing the antineoplastic action of the chemotherapeutic drug.