We have studied the effect of a daily supplement of 2.4 omega-3 fatty acids (omega 3 FAs) to 16 healthy men on acute collagen-induced thrombus formation in flowing non-anticoagulated blood. The supplement was formulated as Triomar capsules, containing 60% omega-3 FAs with an eicosapentaenoic/docosahexaenoic acid ratio of 3/2. A parallel-plate perfusion chamber device was used to study thrombus formation prior to and after 3 months of omega-3 FAs supplement. The wall shear rates at the thrombogenic surface were 650, 2,600 and 10,500 s-1, which are typical for small arteries, slightly stenosed arteries and severely stenosed arteries, respectively. For the latter situation a parallel-plate perfusion chamber with an eccentric stenosis occluding 80% of the cross-sectional area of the blood flow channel was used. The dietary supplement of omega-3 FAs did not cause significant changes in platelet adhesion to collagen or in thrombus volume. However, fibrin deposition was reduced by 34% (P < 0.03) at the highest shear condition (stenosis). Plasma fibrinogen was reduced by 18% (P < 0.0006). Changes in serum concentration of triglycerides, total-cholesterol, LDL- and HDL-cholesterol were not significant. Our data suggest that a moderate intake of omega-3 FAs provides virtually no protection against acute platelet-dependent thrombus formation, irrespective of the shear conditions. However, the significant reduction in plasma levels of fibrinogen following dietary supplementation of omega-3 FAs may be important, since high levels of fibrinogen is associated with cardiovascular disease and thrombosis.