Reports of the effects of trans fatty acids on coronary heart disease are inconsistent. Trans fatty acids may particularly influence coronary risk when linoleic acid levels are low, a situation which occurs in Scotland where prevalence of coronary heart disease is also very high. The link between trans fatty acid intake and prevalent coronary heart disease was therefore investigated in the Scottish Heart Health Study population. Trans fatty acid intakes were calculated from 10/359 sets of food-frequency questionnaire data obtained from the cross-sectional survey of men and women aged 40-59 years. Logistic regression analysis was used to calculate the odds ratios for prevalent coronary heart disease by fifths of dietary intake of total, natural and commercial hydrogenation-derived trans fatty acid. The group who had undiagnosed coronary heart disease at the time of survey was the pertinent group for examining the possible causative effects of trans fatty acid intake. After adjustment for the confounding factors (i.e. age, weight, height, smoking, level of physical activity, blood pressure, total energy intake and intakes of saturated fat, linoleic acid and the antioxidant vitamins) the odds of undiagnosed coronary heart disease for men, relative to the lowest intake fifth, did not differ significantly from unity by total or commercially-derived trans fatty acid intake. Odds were around 35% smaller in the higher intake fifths of naturally-derived trans fatty acids. For women, the odds of undiagnosed coronary heart disease tended to be greater in the higher fifths of total (odds ratio 1.36 (95% confidence interval 0.94, 1.89)) and hydrogenated (1.26 (0.92, 1.72)) trans fatty acid relative to the lowest fifth, but only reached significance in the third fifth of total trans fatty acid (1.36 (1.01, 1.83)). Dietary total and commercially-derived trans fatty acids failed to influence the odds of coronary heart disease for men, even though a significant increase in the ratio of low density plus very low density lipoprotein to high density lipoprotein-cholesterol occurred with trans fatty acid intake. The results, therefore, do not support a major effect of dietary trans fatty acid from commercial hydrogenation on coronary heart disease risk in these Scottish men. The results for women are less clear, and the possibility remains that individuals at the high extreme of trans fatty acid intake, who may be essential fatty acid deficient, are at enhanced risk of coronary heart disease.