The relation between the renal handling of phosphate, expressed as the maximal tubular reabsorption of phosphate (TmP)/glomerular filtration rate (GFR) index, and the serum concentration of immunoreactive parathyroid hormone (i-PTH) has been ivestigated in 15 patients with a very wide range of GFR, TmP/GFR and i-PTH. Seven patients had well functioning kidney allografts, with GFR ranging from 43.1 to 64.9 ml/min, while eight had varying degrees of chronic nephropathy, with GFR ranging from 26.7 to 2.3 ml/min. The TmP, the i-PTH concentration, the 51Cr EDTA clearance, the extracellular volume and the serum concentrations of calcium and standard bicarbonate were estimated during conditions where tubular reabsorption of phosphate was maximal. An inverse significant correlation was demonstrated between TmP/GFR and i-PTH (p less than 0.001), while none of the other investigated factors correlated to the TmP/GFR index. It is therefore concluded that the parathyroid hormone has a key role in the regulation of the tubular handling of phosphate in patients with impaired renal function.