Transplacental feto-maternal hemorrhage was diagnosed after delivery of a highly anemic infant from a healthy mother with polyhydramnios by demonstration of fetal red cells in the maternal circulation (145 degrees/00) using a modified Kleihauer technique. Other causes for anemia such as hemolysis, coagulopathies, failure of red cell production or perinatal posttraumatic bleeding were excluded. The transfused volume estimated eexceeded the blood volume of the newborn by several times, thus indicating a chronic blood loss. This was confirmed by hematologic data typical for a normovolemic anemia, a low serum iron level and the development of a congestive heart failure in the newborn infant.