Listeria monocytogenes, a worldwide pathogen, causes significant perinatal mortality and morbidity and has been implicated in spontaneous abortions, stillbirths, premature delivery, and neonatal sepsis, often with meningitis. Maternal symptoms are frequently minimal, and diagnosis is made only if the suspicion is high and diagnostic maternal blood or amniotic fluid cultures are performed. Because cultures are not routinely performed on spontaneously aborted fetuses, many authors feel that the true incidence of the disease may be underestimated. To date, the absence of a test to retrospectively diagnose Listeria infection has contributed to the lack of accurate estimates of the incidence of the disease. Seven cases in which immunohistochemical stains were used to confirm the diagnosis of placental listeriosis are described. All placentas showed the characteristic lesions with severe chorioamnionitis, numerous microabscesses, and focal necrotizing villitis. Immunohistochemical localization of Listeria antigen was made to the amnion (focally in areas with no inflammatory infiltrate), the abscesses, and the areas with villitis. In general, the antigen was extracellular and intracellular, predominantly within macrophages or the amnion epithelium. Listeria antigen was often found where definite identification of the organism was not possible on Brown-Hopps or Warthin-Starry stains. The immunohistochemical technique may therefore show an increase in sensitivity of detection of L monocytogenes compared with routine bacterial stains. Moreover, the ability to retrospectively evaluate placental specimens for evidence of this organism should permit the true incidence of perinatal listeriosis to be determined.