RhD alloimmunization remains a threat to 1% of the 10% of RhD-negative women in the United States who are giving birth to RhD-positive fetuses despite routine antenatal and postpartum administration of Rh(o)D immune globulin (RhIG). This report examines the clinical course of an RhD-negative woman who developed a high anti-D antibody titer during her pregnancy while carrying an RhD-positive female fetus yet had a negative antibody screen at the time she gave birth. Although she delivered a healthy newborn unaffected by hemolytic disease, subsequent pregnancies will be treated as though she is RhD alloimmunized. The discussion below includes possible causes for the abrupt rise in this woman's anti-D antibody titer, a review of the complex Rh system and cellular anamnestic response, and current fetal surveillance for hemolytic disease of the fetus and newborn.