Twenty-two patients with stable coronary heart disease were randomly assigned to either of two groups. Group I (n = 11) was given acetylsalicylic acid (ASA) 300 mg daily for 1 week, whereafter a daily supplement of 3,4 g eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) as a highly concentrated ethylester, formulation ("K-85", Norsk Hydro) was added for another 4 weeks. Group II (n = 11) was given 3,4 g daily of EPA and DHA for 4 weeks, after which ASA 300 mg daily was added for another week. Determination of serum fatty acids confirmed satisfying absorption of EPA and DHA. A significant increase of the Ivy bleeding time was registered following administration of both "K-85" (240 to 270 sec, median values) and ASA (270 to 360 sec, median values) alone. A slighter increase was noted by a combination of the two principles. A reduction in serum triglycerides of 17% was noted after "K-85" (median values, both groups). Serum total cholesterol decreased after "K-85" administration in group I, but not so in group II. HDL-cholesterol remained unchanged. Serum lipids remained unaffected by ASA. During administration of "K-85" no adverse effects or bleeding episodes were seen.