We have compared the effects of partially hydrogenated fish oil (PHFO diet), partially hydrogenated soybean oil (PHSO diet), and butterfat (butter diet) on fibrinolytic and coagulation variables in 31 young men. The three test margarines, which contributed 78% of total fat in the diets, contained 70% butterfat, PHSO, or PHFO, each with 30% of soybean oil. Fat provided approximately 35% of energy, and the content of trans-fatty acids was 0.9%, 8.5%, and 8.0% of energy in the butter diet, PHSO diet, and PHFO diet, respectively. All diets contained 420 mg cholesterol per 10 megajoules per day. All subjects consumed all three test diets for 3 weeks, in a random order (crossover design). The PHSO diet resulted in higher levels of plasminogen activator inhibitor type 1 antigen and plasminogen activator inhibitor type 1 activity than the two other test diets. Fibrinogen increased on the butter diet compared with the PHFO diet. No significant differences in the levels of factor VII, fibrinopeptide A, D-dimer, tissue plasminogen activator or beta-thromboglobulin were observed between the three test diets. The PHFO and the PHSO diets have previously been shown to result in higher levels of Lp(a) compared with the butter diet. The present findings indicate that PHSO has unfavorable antifibrinolytic effects relative to PHFO and butter and that butter may be procoagulant relative to PHFO. More controlled studies are needed to assess definitely the impact of different hydrogenated fats on risk of coronary heart disease.