The effect on plasma antithrombin III (AT III) and protein C on a supplement with polyunsaturated fatty acids (PUFA's) was investigated in a double-blind study in 36 patients with stable angina pectoris. All participants were given a supplement to their normal diets of vegetable oil (4.8 g n-6 PUFA's) for 4 weeks and were then randomized to the same oil or to fish oil (4.8 g n-3 PUFA's) for 12 weeks. Both oil supplements resulted in a statistically significant decrease in AT III activity, but there were no differences between the two different types of PUFA's. Antithrombin III antigen, protein C antigen or activity did not change significantly after either oil supplement. The background and significance for the decrease in antithrombin III activity induced by n-3 and n-6 PUFA's in patients with ischaemic heart disease is unknown.