It is well established that long chain n‐3 polyunsaturated fatty acids have cytotoxic effects on human breast cancer cells. To determine synergy between n‐3 fatty acids and chemotherapy drugs, we examined the effects of docosahexaenoic acid (DHA) alone and in combination with doxorubicin (DOX) on growth of MDA‐MB‐231 and MCF‐7 human breast cancer cell lines, using the WST‐1 assay. The dose/concentration associated with 50% inhibition (IC50) was determined for DHA and DOX (individually) on both cell lines. After treatment (48h) with DHA at its IC50 concentration (320 μM for MCF‐7, 87 μM for MDA‐MB‐231), cells were treated with DOX at its IC50 (5.1 μM for MCF‐7, 0.3 μM for MDA‐MB‐231); resulting in increased inhibition (compared to individual treatments). Incubation with DHA significantly increased DHA content in phospholipids of both cell lines. Isobologram analysis (using the Calcusyn program) indicated a synergistic relationship for DHA in MCF‐7 cells, and an additive relationship for DHA in MDA‐MB‐231 cells. Our results suggest that incorporation of DHA into two different breast cancer cell lines can improve the cytotoxic effects DOX which is a cornerstone of breast cancer chemotherapy. Canadian Breast Cancer Foundation: Prairies/NWT