Omega-3 fatty acids (n-3 FA) may reduce atherogenesis and thrombosis. We investigated the effects of n-3 FAs on procoagulant activity and cytokine expression in whole blood cultures from patients with atherosclerosis. Eleven of the 23 included patients had received 5.1 g n-3 FA daily for 6 months (group I) whereas 12 patients had been on placebo (group II). All patients were then given 5. g n-3 FA daily for another 4 weeks. At baseline significantly lower levels of LPS-induced prothrombin fragment1+2 were found in group I (p = 0.010), this difference being eliminated after 4 weeks. Il-6 and TNFalpha were significantly higher at baseline in group I and the differences in changes from baseline between the groups were statistically highly significant with increasing values in group II(Il-6 p = 0.001, TNF alpha p = 0.002). The present results indicate a reduction in pro-thrombotic potential in patients receiving highly concentrated n-3 FA, whereas some proinflammatory responses might be adverse.