We describe two cases of congenital neuroblastoma involving the placenta and review previously reported cases. The placentae in congenital neuroblastoma have a bulky, hydropic appearance, and contain tumour cells which are confined to the fetal circulation. The tumour emboli are not macroscopically identifiable. Pathophysiological mechanisms of placental involvement by fetal and maternal malignancies are considered. The rarity of this lesion may be artefactual, and may result from failure to examine grossly enlarged placentae in cases of stillbirth and hydrops fetalis. Congenital malignancy must be considered in the differential diagnosis of an abnormally large placenta.