Objective To determine whether endothelial cell injury would be produced by factor(s) released into the fetal circulation, manifested by altered messenger RNA expression of nitric oxide synthase. Design Case -control study.Setting University teaching hospital.Samples Fetal plasma was collected from 34 normal pregnancies, 44 pregnancies with umbilical placental vascular disease identified by an abnormal umbilical Doppler and 11 pregnancies with maternal pre-eclampsia but with normal umbilical Doppler studies. Methods Aliquots from a common culture of human umbilical vein endothelial cells (HUVECs) were incubated with fetal plasma from the members of the three patient groups. The total RNA was extracted from the endothelial cells and mRNA for nitric oxide synthase was measured by reverse transcription and semi-quantitative polymerase chain reaction (RT-PCR). This was standardised by comparison of the amplified inducible nitric oxide synthase (iNOS) or endothelial constitutive nitric oxide synthase (ecNOS) to glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Main outcome measure Endothelial cell gene expression of iNOS and ecNOS.Results The mRNA expression of iNOS and ecNOS were significantly higher ( P < 0.05) in HUVECs stimulated by fetal plasma from pregnancies with umbilical placental vascular disease [iNOS 1.12 (0.16); ecNOS 1.78 (0.18)] when compared with normal pregnancies [iNOS 0.56 (0.06); ecNOS 1.06 (0.10)]. In the maternal pre-eclampsia group, the NOS expression [iNOS 0.76 (0.11); ecNOS 1.39 (0.26)] did not differ from normal pregnancy. In the vascular disease group, there was no difference in NOS expression between the subgroups with and without maternal pre-eclampsia. Conclusions Our study demonstrates that umbilical placental vascular disease is associated with a factor(s) in fetal plasma that produces an increase in the expression of iNOS and ecNOS mRNA by endothelial cells. Our findings raise the possibility that the release of factors causing an up-regulation of iNOS and ecNOS in the endothelium in the fetal placenta may occur as part of an inflammatory response of the vascular endothelium to injury.