BACKGROUND: Studies of long-term intake of industrially produced trans fatty acids (TFA) and n-3 polyunsaturated fatty acids (PUFA) suggest opposite effects on cardiovascular disease risk. Common mechanisms of action are probable. OBJECTIVE: To examine the effects on cardiovascular risk markers of dietary enrichment with TFA or n-3 PUFA. DESIGN: Randomized, double-blind, parallel intervention trial. SETTING: Department of Human Nutrition, The Royal Veterinary and Agricultural University. SUBJECTS: In all, 87 healthy males included, 79 completed. INTERVENTION: Subjects were randomly assigned to 8 weeks of a daily intake of 33 g of experimental fats from either partially hydrogenated soy oil containing 20 g of TFA, 12 g of fish oil with approximately 4 g of n-3 PUFA and 21 g of control fat, or 33 g of control fat. The experimental fats were incorporated into bakery products. Plasma lipids, blood pressure, heart rate variability (HRV), arterial dilatory capacity, compliance, and distensibility were recorded before and after intervention and at follow-up 12 weeks after the intervention. RESULTS: High-density lipoprotein cholesterol (HDL-C) decreased in the TFA group and triglycerides and mean arterial blood pressure decreased in the n-3 PUFA group compared to the control group. HRV, arterial dilatory capacity, compliance, and distensibility were unchanged. CONCLUSION: The results indicate that the association between coronary heart disease risk and intake of TFA and n-3 PUFA relates only modestly to changes in traditional risk markers. SPONSORSHIP: Danish Medical Research Council (Grant no. 22-01-0390), Center of Advanced Food Research (Copenhagen, Denmark) (Grant no. KVL-R-2001-107), the Danish Heart Association (Grant no. 99-2-3-45-22748), Novozymes (Bagsvaerd, Denmark), Aarhus Olie (Aarhus, Denmark), and from private sources. The experimental fats were provided by Pronova Biocare (Aalesund, Norway) and Aarhus Olie (Aarhus, Denmark).