We describe a 29-year-old male with untreated primary neonatal hyperparathyroidism. Hypotonia, poor feeding, failure to thrive, and developmental delay were noted in early infancy and in incidental serum calcium of 3.8 mmol/L was dismissed as a laboratory error. Childhood was characterized by profound muscle wasting and progressive spastic quadriparesis. Distinctive skeletal deformities, facial dysmorphism, and perichondral calcifications are now evident in adulthood. Elevated serum calcium (range: 2.7-3.3 mM; normal less than 2.7 mM), serum immunoreactive parathyroid hormone (range: 1,405-1,817 pg/mL; normal 50-140 pg/mL), and markedly decreased urinary calcium excretion (0.04 mumol/dL glomerular filtrate; normal greater than 25) suggested the diagnosis of primary neonatal hyperparathyroidism. This was supported by evidence of hypocalciuric hypercalcemia--the autosomal dominant carrier state--in both the parents. Our case illustrates the profound neurodevelopmental deficits arising from sustained hypercalcemia in infancy and childhood. Although this disorder is not lethal, it should be considered a neonatal emergency, since surgical parathyroidectomy can result in cure.