Human diet contains large amounts of trans fatty acids originating primarily from hydrogenated fats. Consumption of trans fatty acids is considered safe for man, but side effects, including impaired biosynthesis of long-chain polyunsaturated fatty acids with 20 and 22 carbon atoms and reduced growth have been observed in animals. We studied whether or not there are indications of untoward effects of trans fatty acids in 29 premature infants (birth weight 1700 +/- 127 g, gestational age 33.6 +/- 1.4 weeks, mean +/- SD). Plasma samples obtained on day 4 of life were analysed for fatty acid composition. Trans octadecenoic acid and total trans fatty acids in plasma lipid fractions (% wt/wt) were not related to the precursor essential fatty acids linoleic and alpha-linolenic acids but correlated inversely to n - 3 and n - 6 long-chain polyunsaturated fatty acids and to the product/substrate ratios of long-chain polyunsaturate biosynthesis. Trans fatty acids were also inversely correlated to birth weight but not to gestational age. These data indicate a potential impairment of essential fatty acid metabolism and early growth by trans isomers in man, and question the safety of high dietary trans isomer intakes during pregnancy and the perinatal period.