Trans isomers of α-linolenic acid, which are formed by deodorization of refined vegetable oils, can be found in significant amounts in edible oils. Effects of trans α-linolenic acid on plasma lipoproteins are unknown. We therefore investigated the effects of trans α-linolenic acid on plasma lipids and lipoproteins in healthy European men. Eighty-eight healthy men from three European countries (France, Scotland, UK and the Netherlands) first consumed for 6 weeks a diet with experimental oils ‘free’ of trans fatty acids (run-in period). For the next 6 weeks, they were randomly allocated to a diet with experimental oils ‘high’ or ‘low’ in trans α-linolenic acid. Daily total trans α-linolenic acid intake in the high trans group was 1410 (range 583–2642) mg. Experimental oils were provided as such, or incorporated into margarines, cheeses, muffins and biscuits. The high trans α-linolenic acid diet significantly increased the plasma LDL-:HDL-cholesterol ratio by 8.1 % (95 % CI 1.4, 15.3; P=0.02), and the total cholesterol:HDL-cholesterol ratio by 5.1 % (95 % CI 0.4, 9.9; P=0.03) compared with the low-trans diet. This was largely explained by an increase in LDL-cholesterol on the high-trans diet, while no change was observed in the low-trans group (mean treatment effect of 4.7 % (95 % CI -0.8, 10.5; P=0.10). No effects were found on total cholesterol and HDL-cholesterol, triacylglycerols, apolipoprotein B and A-1, and lipoprotein(a) concentrations. In conclusion, trans α-linolenic acid may increase plasma LDL-:HDL-cholesterol and total cholesterol:HDL-cholesterol ratios. Whether diet-induced changes in these ratios truly affects the risk for CHD remains to be established.