The mechanism of the increased phosphaturia of chronic renal failure was investigated in seven patients with creatinine clearances ranging from 22 to 63 ml/min. Phosphorus deprivation for 2 to 7 weeks resulted in a marked and rapid reduction in urinary total and fractional phosphate excretion. Serum immunoreactive parathyroid hormone concentration initially remained unchanged and eventually decreased slowly from 330 +/- 50 microliterEq per ml (control) to 252 +/- 58 microliterEq per ml (P less than 0.025), but persisted substantially elevated above the normal range (10-60 microliterEq per ml). Thus, phosphate excretion in chronic renal failure can be regulated to a major extent by the dietary phosphorus intake independently of parathyroid hormone.