Trans unsaturated fatty acid originated from synthetic technology. Human consumption of trans isomers of unsaturated fatty acids increased markedly. The induction of growth failure was observed in animals when trans fatty acids were supplied. We studied whether or not there are indications of untoward effects of trans fatty acids in premature infants. Trans fatty acids were in higher concentrations in plasma of 23 women in pre-term delivery compared with 21 women delivering in normal term. Trans fatty acids in plasma were also inversely correlated to birth weight and with gestational age. These data indicate a impairment of intrauterine growth by trans isomer of unsaturated fatty acids.