Our aim was to investigate the expression of S100B protein in the amnion and to assess the amniotic fluid concentration in pregnancies complicated by pre-eclampsia. Samples were obtained from women who developed pre-eclampsia (n = 7), pre-eclampsia with intrauterine growth retardation (IUGR) (n = 4), normotensive IUGR (n = 7) and gestational hypertension (n = 4) during pregnancy and healthy controls who delivered at term (n = 35). To determine the difference in the expression of S100B in the amnion, we performed immunohistochemistry, western blot analysis and RT-PCR. Using enzyme-linked immunosorbent assay (ELISA), we assessed the S100B concentration in amniotic fluid. The S100B mRNA expression in the amnion of pre-eclamptic patients and patients with pre-eclampsia with IUGR was significantly higher than that in the control. The amniotic fluid S100B protein concentration of the pre-eclampsia and normotensive IUGR cases was significantly higher than that of the control. This study shows that amnion could be a source responsible for the increased concentration of S100B in amniotic fluid. In pre-eclampsia, reactive oxygen species (ROS) are generated by oxidative stress. Some pathological conditions that develop during pregnancy and are related to hypoxic stress can affect the elevation of S100B concentration in the amnion.