Trans isomeric fatty acids disturb themetabolism of essential polyunsaturated fatty acids in animals and in premature infants. We assessed whether similar effects may also occur in healthy children. Plasma phospholipid fatty acid composition was analysed in 53 apparently healthy children aged 1-15 years (mean 7.5 years). Trans fatty acids were found in all samples and contributed 1.78 ± 0.10% (w/w, mean ± SEM). There was no relation of the major trans isomer octadecenoic acid and of total trans fatty acids to the precursor essential fatty acid linoleic acid. In contrast, we found significant inverse correlations of trans octadecenoic acid and total trans isomers to the principal n-6 metabolite arachidonic acid as well as to the sum of all n-6 metabolites. Furthermore, there was an inverse correlation of total trans fatty acids to the ratio of arachidonic to linoleic acid, an indirect indicator of linoleic acid conversion. These findings are compatible with inhibition of arachidonic acid biosynthesis by trans fatty acids. Since the availability of long-chain polyunsaturated fatty acids, including arachidonic acid, is of essential importance for tissue growth and development, these findings question the safety of high dietary trans fatty acid intakes in childhood.