In a randomized, placebo-controlled study the effect of 10 weeks of supplementation with either 5.2 g of a concentrated fish oil triglyceride (Triomar) enriched in omega-3 fatty acids (1.60 g/day EPA and 1.04 g/ day DHA) or 5.2 g corn oil (serving as placebo) on maximal aerobic power, anaerobic threshold and running performance was assessed in 28 well-trained male soccer players (18-35 years). Supplements were given as 650-mg capsules. Capsule assignment was randomized to one omega-3 group (n = 15), given eight Triomar capsules per day, and one placebo group (n = 13), given eight capsules of corn oil per day. During the 10-week supplementation period the subjects maintained their usual diets and training regimes. Red blood cell (RBC) osmotic fragility, triglycerides and fatty acid composition in plasma were assessed before and after the supplementation period. The pre- and post-supplementation tests of maximal aerobic power, anaerobic power and running performance showed no significant difference between the two groups. Subjects in the omega-3 group had significantly reduced plasma triglycerides, rose EPA (175%) and DHA (40%) in the total lipid fraction of plasma after supplementation. RBC osmotic fragility did not change. In conclusion, the results do not support the hypothesis that endurance athletes can improve maximal aerobic performance by omega 3-fatty acid supplementation.