We studied the effect of incorporation of eicosapentaenoic and docosahexaenoic acid into the normal Western diet in nine human volunteers for 6 weeks. They received 1.5 g per day of omega 3-polyunsaturated fatty acids (W3-PUFA) for 3 weeks followed by 3 weeks with supplementation of 3.0 g per day. A control period of 3 weeks preceded PUFA supplementation. A significant effect on lipids was found in the total triglycerides and VLDL-triglyceride fractions (p less than 0.025). The ratio VLDL-triglyceride over HDL-cholesterol was also significantly diminished after 6 weeks of treatment (0.86 +/- 0.15 vs 1.01 +/- 0.19, p less than 0.02). Prolongation of the bleeding time was clearly demonstrated after 6 weeks of supplementation (10.6 +/- 1.3 min vs 6.2 +/- 0.6 min). Platelet aggregation stimulated by either ADP or collagen was significantly diminished after W3-PUFA supplementation (p less than 0.025 and p less than 0.005 after ADP and collagen respectively). The lag time of platelet aggregation was significantly prolonged (94 +/- 12 vs 56 +/- 2 sec, p less than 0.005) and the maximum speed reaction of the primary phase of the reaction was significantly diminished (1.3 +/- 0.2 vs 1.6 +/- 0.2 mm/sec, p less than 0.02). In conclusion, in the light of this study W3-PUFA supplementation leads to two different benefits in cardiovascular prevention. First, a significant diminution of total plasma triglyceride has been shown and, second, prolongation of the bleeding time and diminution of the platelet aggregation have been demonstrated.