Two decades of research conclusively demonstrated the antithrombotic properties of the long-chain n-3 polyunsaturated fatty acids (PUFA) present in lipids from marine fishes. Most American consumers, however, given their preference for meat, will not realize the benefits of a fish-rich diet. Could alpha-linolenic acid (18:3, n-3) be similarly effective via modulation of the synthesis of vasoactive eicosanoids, i.e., thromboxane and prostacyclin? The present pilot study is a contribution toward answering this question. We determined that the urinary excretion of 11-dehydrothromboxane B2 declined by 34% from baseline level 7 weeks after the n-6/n-3 ratio of dietary PUFA was reduced from 28:1 to 1:1. The excretion of 2,3-dinor-6-oxo-prostaglandin F1 alpha was similarly affected. The dietary adjustment was brought about by substituting measured amounts of canola and flaxseed oils (3:1) for measured amounts of olive and corn oils (3:1) in an otherwise fat-free basal diet. This study demonstrates that dietary alpha-linolenic acid is an effective modulator of thromboxane and prostacyclin biosynthesis. Therefore, we can expect that the eicosanoid-mediated effects of alpha-linolenic acid are similar to those elicited by marine lipids.