Parathyroid function is described as normal in patients with phosphopenic rickets before initiation of therapy with phosphate salts; however, hyperparathyroidism is an occasional complication of treatment. We observed a higher than expected frequency of hyperparathyroidism in patients with phosphate-wasting rickets, present before treatment in some patients and, more frequently, after the onset of treatment. To better define parathyroid status in hypophosphatemic rickets, we sampled 12 affected children and 7 affected adults every 4 h for 1 day and measured PTH in assays detecting midmolecule fragments (cPTH) and intact hormone (iPTH). All children and 4 adults were receiving a vitamin D preparation and phosphate salts; 3 adults were untreated. Mean cPTH, iPTH, and nephrogenous cAMP excretions in each group of patients were greater than in controls. Exaggerated nocturnal rises in both cPTH and iPTH characterized the profile in patients. Seventeen patients demonstrated frankly elevated cPTH at night, with peak values at midnight, whereas no control individual did. Although mean iPTH values in patients increased at night, they did not exceed the upper limit of normal. Hyperparathyroidism in hypophosphatemic rickets occurs in both children and adults, may be present in untreated patients, is predominantly nocturnal, and is characterized by exaggerated secretion of midmolecule fragments. This manifestation of hypophosphatemic rickets is more widespread than currently recognized; we speculate that it may contribute to the pathogenesis of nephrocalcinosis and precede the development of tertiary hyperparathyroidism.