ABSTRACT To determine whether parathyroid hormone is required for renal response to phosphorus loading, the effects of ingestion of 3100 mg of phosphorus daily for 3 days were compared in 5 treated hypoparathyroid patients and 5 normal control subjects of comparable age and sex. In both normal and hypoparathyroid subjects the response to phosphorus loading was characterized by an increase in urinary excretion of phosphorus without a concomitant increase in the rate of glomerular filtration of phosphorus. Some mechanism other than changes in rates of parathyroid hormone secretion probably accounts for renal responses to changes in phosphorus intake. The timing of urine and serum collections with regard to food intake and the total amount of phosphorus in the diet were found to affect the assessment of renal handling of phosphorus. The finding that a 3-day period of oral phosphorus loading did not increase the serum phosphorus level in the hypoparathyroid patients suggests that phosphorus restriction is not always necessary in the treatment of hypoparathyroidism. The addition of one more condition to the list of those that may affect the percentage of renal tubular resorption of phosphorus in no way decreases the usefulness of this determination in the diagnosis of hyperparathyroidism.