Dietary trans fatty acids are associated with increased risk of CHD. We hypothesized that the changes in plasma lipids associated with a high intake of trans fatty acids would cause adverse effects on procoagulant and fibrinolytic activities. A randomized crossover controlled feeding study was conducted in twenty-nine men. A trans-rich diet supplying 10 % energy as trans- 18:1 was compared with diets in which the trans fatty acids were replaced either with carbohydrate or oleate; each diet was taken for 2 weeks in random order. Fasting fibrinogen and D-dimer concentrations and factor VII coagulant, plasminogen activator inhibitor type 1 and tissue plasminogen activator did not differ between diets. Postprandially, tissue plasminogen activator activity increased and plasminogen activator inhibitor type 1 activity decreased on all diets. Factor VIIc increased postprandially by 15 and 17 % on the trans and oleate diets respectively, compared with an 11 % increase on the carbohydrate diet; the mean difference between oleate and carbohydrate diets was 6 (95 % CI 0·2, 11·9) %. The LDL-cholesterol:HDL-cholesterol and apolipoprotein B: apolipoprotein A-I ratios increased by 13 (95 % CI 5·7, 21·8) and 10 (95 % CI 3·1, 17·2) % respectively on the trans diet compared with the oleate diet and by 6 (95 % CI 0·1,12·7) and 7 (95 % CI 0, 13·5) % respectively compared with the carbohydrate diet. Plasma HDL2-cholesterol concentration was 18 (95 % CI 0·7, 35·9) % lower on the trans diet compared with the oleate diet. The results confirm adverse effects of trans fatty acids on HDL-cholesterol concentrations, but suggest that trans fatty acids do not have any specific effects on known haemostatic risk markers for cardiovascular disease in healthy young men in the short-term.