Complete hydatidiform moles are entirely paternally derived and, therefore, represent a complete intrauterine allograft that might be expected to provoke an altered maternal immune response compared with that of normal pregnancy. Uterine decidua contains a large leukocyte population, of which 10%–20% are T lymphocytes. Fas ligand (FasL) expression by placental trophoblast may induce apoptosis of Fas lymphocytes, thereby facilitating immune tolerance and survival of the molar trophoblast. Our previous studies have shown an increase in activated CD4 decidual T cells in molar pregnancy compared with normal pregnancy. This study was designed to characterize and quantitate Fas/FasL expression by decidual leukocytes in complete and partial hydatidiform mole compared with that in normal early pregnancy using single and double immunohistochemical labeling (i.e., avidin-biotin-peroxidase and avidin-biotin-alkaline phosphatase). A significant increase was found in Fas and FasL expression by decidual CD4 T cells in complete (Fas , P = 0.0106; FasL , P = 0.0081) and partial (Fas , P = 0.0131; FasL , P = 0.0051) hydatidiform moles, as was a significant decrease in Fas expression by decidual CD8 T cells in complete (P = 0.0137) and partial (P = 0.0202) hydatidiform mole compared with normal early pregnancy. The implications of altered Fas/FasL status of decidual T-cell subsets in hydatidiform mole are also discussed.