Hydrogenation of vegetable oils produces fatty acids with unusual structures having trans double bonds and double bonds in new positions of the acyl chain. This study was designed to determine which of these fatty acid isomers are incorporated or accumulated in humans during long-term dietary consumption of hydrogenated fats. The double bond position and configuration of the octadecenoate fraction of total lipids extracted from human heart, brain, liver, aorta, and adipose tissue were determined. The level of trans octadecenoate in the tissues as determined by both direct gas-liquid chromatography (GLC) and by GLC after silver nitrate thin-layer chromatography ranged between 0.4 and 5.0%, with an average of 2.7%. Tissues were found to contain trans-octadecenoic isomers having double bonds between the 6 and 15 positions, whereas cis double bonds were found to occur between the 6 and 14 positions. The distribution of double bonds in adipose tissue correlated very closely with the composition of dietary hydrogenated fat. Thus, essentially all of the unusual octadecenoic fatty acid isomers that are produced during vegetable oil hydrogenation are incorporated into human tissue. However, in contrast to results of short-term (1-6 months) feeding studies of animals, our results suggest that long-term (20-60 years) consumption of hydrogenated fats by humans does not lead to substantial preferential accumulation of positional isomers in human tissue total lipids.