Massive fetomaternal hemorrhage has been associated with numerous risk factors and can occur spontaneously. The risk is probably greater when a woman who is Rh negative has an ABO-compatible fetus. We report the case of a gravid, A-negative unsensitized patient who came to the hospital at term complaining of decreased fetal movement over the previous 24 hours. During evaluation, the fetal heart rate was found to have decreased beat-to-beat variability and repetitive late decelerations, and the mother was delivered of a 3,005 gA-positive neonate (by cesarean section) with a hemoglobin level of 2.9 g/dL. An acid elution test showed 400 mL of fetal blood in the maternal circulation, and the patient received 23 ampules (6,900 micrograms) of Rh immune globulin postpartum. The patient's condition was observed for 156 days after delivery; she did not become sensitized to the Rh factor. A massive fetomaternal hemorrhage can occur without any antecedent risk factors, with a risk of subsequent morbidity to the neonate. Sensitization can be prevented by prompt administration of adequate amounts of Rh immune globulin.