This study was carried out to determine the frequency and to quantitate the severity calcium-phosphate deposits in end-stage kidneys. In 57 of 59 end-stage kidneys obtained from patients with a variety of different renal diseases, calcium levels were greater than 2 standard deviations (SD) above control values. The mean calcium concentration was 157 +/- 24 mmol/kg dry defatted tissue in the end-stage kidneys as compared to 17 +/- 1 mmol/kg in the control kidneys. Histologically, calcium was deposited in the cortical tubular cells, basement membranes and interstitium. It would appear that calcification occurred during the course of renal failure rather thant terminally in that the kidney calcium concentration bore no relationship to the calcium X phosphate product, and the calcium concentration in the kidneys of uremic patients undergoing dialysis (144 +/- 23 mmol/kg) was no greater than that found in uremic patients not undergoing dialysis (188 +/- 62 mmol/kg). It is suggested that calcification may damage the diseased kidney accelerating the rate of renal functional deterioration.