We report three neonates with transient hypoparathyroidism with elevated parathyroid hormone (PTH) levels to clarify further the pathogenesis of late neonatal hypocalcemia and calcium homeostasis. Clinical signs were seizures starting at age of 10 and 11 days. The biochemical features were characterized by transient hypocalcemia and hyperphosphatemia due to a high transport maximum of the phosphate/glomerular filtration rate, despite high PTH levels. All had normal magnesium and calcidiol levels (at least 5 micrograms/l) for their age, and this precludes hypoparathyroidism due to low magnesium levels and hyperparathyroidism due to overt vitamin D deficiency. To diagnose pseudohypoparathyroidism type I, intravenous human PTH (1-34) infusions were performed; however, they showed brisk responses of plasma and/or urine cyclic AMP in response to the PTH infusion, but the phosphaturic response to the PTH was sluggish compared to the controls. All three showed an increase in serum alkaline phosphatase activity, suggesting PTH stimulation of osteoblasts. They were treated initially with calcium lactate or (1 alpha)-hydroxycalciol/calcitriol. Their hypoparathyroid condition, however, was transient; they maintained normal serum calcium and PTH levels without medication before the age of 6 months. The etiology, possibly intracellular signal transduction distal to cyclic AMP and/or distinct from adenylate cyclase in the kidney, is developmental and the condition was resolved completely within 6 months of age. We have termed this condition "transient pseudohypoparathyroidism of the neonate".