The Fas-Fas ligand (FasL) pathway of apoptosis is abnormally activated in diseases associated with impaired immune tolerance or chronic inflammation. Pregnancy-related hypertension is a spectrum of disease that commonly causes significant morbidity in women and in their newborn infants, is associated with generalized inflammation, and may be causally related to impaired maternal-fetal tolerance. Our recent observation of enhanced trophoblast expression of FasL in one form of pregnancy-related hypertension led us to hypothesize that this group of disorders might be associated with abnormal activation of the Fas-FasL pathway. To test this hypothesis, we prospectively quantified soluble and leukocyte-associated Fas receptor and FasL in the maternal and umbilical cord blood (CB) sera of 20 gestations complicated by preeclampsia and of 18 normal control gestations, using ELISA and flow cytometric analyses. We determined higher soluble FasL levels in paired maternal and CB sera of hypertensive gestations compared with control gestations (p Ͻ 0.01); in contrast, soluble Fas levels were similar between groups. Surface expression of FasL was lower on maternal (p Ͻ 0.01) and CB (p Ͻ 0.05) neutrophils from affected gestations, whereas surface Fas expression was lower on maternal (p Ͻ 0.02), but not CB, neutrophils and lymphocytes. We conclude that expression of Fas and FasL in sera and on leukocytes is altered in gestations complicated by preeclampsia, and speculate that activation of the Fas-FasL pathway mediates associated pathologic processes in affected women and in their neonates. Activation of the Fas-Fas Ligand pathway involves cellular interactions between the Fas receptor and FasL, and can result in apoptosis or in an inflammatory response (1). The Fas receptor (CD95, Apo-1, FasR), a type I integral membrane protein belonging to the tumor necrosis factor receptor family, is expressed on numerous cell types including hematopoietic cells, and its expression increases during inflammation (1). In contrast, expression of FasL (CD95L), a type II transmembrane molecule belonging to the tumor necrosis factor receptor superfamily, is limited to certain leukocytes and tissues with immune privilege (2). Fas-FasL interactions leading to clonal deletion of antigen-presenting cells have been implicated in the establishment of graft tolerance (3), and provide one explanation for the immune privilege status of certain tissues, including the placenta, cornea, and testis (2). In contrast, abnormal function of the Fas-FasL pathway is involved in the pathogenesis of a variety of disorders, including cytopenias, autoimmune diseases, graft-versus-host disease, and graft rejection (1,(3)(4)(5)(6).Mice with the gld mutation are deficient in FasL function and have smaller litters with decreased viability (7). Studies in these mice suggest that the Fas-FasL pathway contributes to preservation of the fetal allograft by preventing the trafficking of activated leukocytes between the mother and fetus during normal gestation, observations also ...