Context: Primary hyperparathyroidism with hypercalciuria has not been described in the newborn period. Objective: Our objectives are to identify the genetic basis for neonatal primary hyperparathyroidism in a family with 2 affected children. Subjects: An African American boy presenting with mild neonatal primary hyperparathyroidism and hypercalciuria was evaluated at The Children's Hospital of Philadelphia. His older brother with neonatal primary hyperparathyroidism had died in infancy of multiple organ failure. Methods: We collected clinical and biochemical data and performed exome sequencing analysis on DNA from the patient and his unaffected mother after negative genetic testing for known causes of primary hyperparathyroidism. Results: Exome sequencing followed by Sanger sequencing disclosed 2 heterozygous mutations, c.1883C>A, p.(A628D) and c.2786_2787insC, p.(T931fsX10), in the SLC12A1 gene, which was previously implicated in antenatal type 1 Bartter syndrome. Sanger sequencing confirmed the 2 mutations in the proband and his deceased brother; both parents were heterozygous for different mutations and an unaffected sister was homozygous for wild-type alleles. Conclusions: These results demonstrate a previously unrecognized association between neonatal primary hyperparathyroidism and mutation of SLC12A1, the cause of antenatal Bartter syndrome type 1, and suggest that the loss of sodium-potassium-chloride cotransporter-2 cotransporter activity influences parathyroid gland function. We identified biallelic mutations in the SLC12A1 gene, which was previously implicated in antenatal type 1 Bartter syndrome, in DNA from a boy and his deceased brother, both of whom had neonatal primary hyperparathyroidism.