We report six cases of chronic intervillositis, an infrequently recognized placental lesion that is characterized by a prominent mononuclear inflammatory cell infiltrate in the intervillous space and that is associated with poor fetal outcome. In all six placentas, the inflammatory infiltrate was essentially limited to the intervillous space: chronic villitis was present focally only in one and absent in the other five. Additional placental histopathologic findings included increased villous fibrinoid material in all six, infarcts in two, atherosis in decidual vessels in two, and acute chorioamnionitis in two. Results of immunohistochemical staining confirmed the predominantly histiocytic nature of the intervillous infiltrate. Two mothers had a history of severe preeclampsia, one had elevated blood pressure at the time of delivery, two had a history of substance abuse, two had a history of systemic lupus erythematosus treated with prednisone, and one of these last two also had diabetes. Five of the six pregnancies resulted in perinatal death. One fetus was nonviable, one was anencephalic, one died in utero, and two died of complications of prematurity shortly after birth; one of the premature infants was small for gestational age. The mononuclear nature of the inflammatory cell infiltrate and its association with increased villous fibrinoid material and atherosis suggests an immunological origin, although the possibility that this lesion may have an infectious cause cannot be excluded.