Hyperparathyroidism during pregnancy is clearly associated with an increased incidence of neonatal morbidity and mortality. Although it is impossible to define the precise incidence of this entity, we believe that its occurrence will be seen more frequently with the increasing numbers of female patients who have successfully received renal transplants and with the routine determination of serum chemistries in the nontransplanted pregnant patient. A review of case reports since 1962 of women known to be hyperparathyroid during pregnancy revealed 80 per cent of these pregnancies to be complicated by neonatal tetany, death, or abortion. This review substantiates Ludwig's earlier report [1], which noted a 50 per cent incidence of neonatal complications despite the advances of prenatal and postnatal medical care. There have been only eight reported cases in which parathyroid resection was performed during pregnancy. Successful operation dramatically reduced the incidence of neonatal complications. An adaptive normocalcemic hyperparathyroidism occurs routinely during pregnancy. However, in the hypercalcemic hyperparathyroid pregnancy, transplacental passage of calcium leads to a profound hypercalcemia in the fetus. Since the fetal parathyroid glands are functionally responsive, parathyroid suppression is thought to occur in utero due to high calcium levels. This can lead to neonatal tetany or perhaps permanent neonatal hypoparathyroidism. When a patient presents with significant hypercalcemic hyperparathyroidism during pregnancy, we suggest that an explorative parathyroid operation be performed during the second trimester of pregnancy. After delivery, the baby's course should be carefully monitored with frequent calcium determinations. Cow's milk or other formula feedings high in phosphate content should be avoided in favor of feedings with a calcium:phosphorus ratio similar to that of human milk.