BACKGROUND: Breast cancer becomes lethal when visceral metastases develop. At this stage, anti-cancer treatments aim at relieving symptoms and delaying death without resulting in additional toxicity. On the basis of their differential anti-oxidant defence level, tumour cells can be made more sensitive to chemotherapy than non-tumour cells when membrane lipids are enriched with docosahexaenoic acid (DHA), a peroxidisable and oxidative-stress-inducing lipid of marine origin. METHODS: This open-label single-arm phase II study evaluated the safety and efficacy (response rate), as primary end points, of the addition of 1.8 g DHA daily to an anthracycline-based chemotherapy (FEC) regimen in breast cancer patients (n ¼ 25) with rapidly progressing visceral metastases. The secondary end points were time to progression (TTP) and overall survival (OS). RESULTS: The objective response rate was 44%. With a mean follow-up time of 31 months (range 2 -96 months), the median TTP was 6 months. Median OS was 22 months and reached 34 months in the sub-population of patients (n ¼ 12) with the highest plasma DHA incorporation. The most common grade 3 or 4 toxicity was neutropaenia (80%). CONCLUSION: DHA during chemotherapy was devoid of adverse side effects and can improve the outcome of chemotherapy when highly incorporated. DHA has a potential to specifically chemosensitise tumours.