A high intake of trans fatty acids (TFAs) has been shown to have an undesirable effect on serum lipid profiles and lipoprotein(a) (Lp(a)) levels and may thereby increase the risk for coronary heart disease (CHD). We performed a study in CHD patients, and measured the TFA concentration of the plasma phospholipid fraction. Comparison was made between a case group with angiographically documented severe CHD (> 80% stenosis in one coronary vessel, n = 83) and a control group of patients who had just minor stenosis on the coronary angiography (< 50% stenosis in all three major vessels, n = 78). All subjects were under 68 years of age and were prestratified on age, gender and smoking habits. The two groups were comparable according to the prestratification criteria, body mass index, blood pressure, number of cigarettes smoked and total fat intake. Controls had higher plasma HDL levels (P < 0.001) and lower, albeit not significantly lower, (P = 0.07) plasma LDL levels. No significant correlations were found between percentages of TFAs in plasma phospholipids and plasma LDL or HDL cholesterol levels. Of the major fatty acid classes, only the percentage of saturated fatty acids was significantly higher in cases (46.2 ± 0.92%) than in controls (45.8 ± 1.07% (means ± S.D.)). The difference in total TFA content between cases and controls (0.32 ± 0.02% versus 0.35 ± 0.02%) was -0.03% (P = 0.2). For the specifrc TFAs C16:1n - 7tr, C18:1n - 9tr and C18:2n - 6tr, just minor differences were found. Adjusted odds ratios for tertiles of TFA percentages were 0.56 (0.25-1.23) and 0.76 (0.36-1.61) for the highest and middle tertile compared to the lowest. These findings do not support an association between TFA intake and risk for coronary heart disease.