With the aid of a highly selective capillary column coated with SP 2340, individual trans-fatty acids were identified in all tissue samples studied, the values being unrelated to the degree of atherosclerosis. The mean values of total trans-acids present in human myocardium, jejunum and aorta amounted to less than 1% of the total fatty acids. The trans-acid content was composed of traces of 9-trans-tetradecenoic acid and nearly equal amounts of 9-trans-hexadecenoate and trans-octadecenoate, the double bond in the latter occurring in the delta6, delta9 and delta11 positions. 9-trans,12-trans-octadecadienoate or the corresponding mixed geometric isomers could not be identified in any sample. Additionally, significant increases of 20:4c(n-6), 22:4c(n-6) and 22:6c(n-3) acids were measured in the myocardium of patients who had died from severe atherosclerosis of the aorta and coronary arteries. The atherosclerosis was not attributable to different dietary habits, as judged from the content of linoleic acid in their adipose tissue.