The effect of calcium infusion on serum levels of calcium and phosphorus and on the fractional excretion of phosphorus (Cp/C<sub>cr</sub>) was studied in eight normal subjects and in 35 patients with varying degrees of renal failure. The increment in serum calcium in patients with advanced renal failure and radiographic evidence of osteitis fibrosa was significantly less than that observed in patients with a comparable renal function but without evidence for such bone disease. The increase in serum phosphorus usually observed after calcium infusion in normal subjects was also seen in patients with chronic renal disease but was less marked as renal failure worsened. In patients with advanced renal failure and osteitis fibrosa serum phosphorus fell after calcium infusion. The fall in Cp/C<sub>cr</sub> following calcium infusion in patients with mild renal failure was similar to that observed in normal subjects (50 to 90%), but was diminished in patients with advanced renal failure; in the latter group the maximal decrements in Cp/C<sub>cr</sub> were less than 25% in most patients irrespective of the presence or absence of osteitis fibrosa and overt secondary hyperparathyroidism. The results indicate that in patients with advanced renal failure: (1) the usefulness of changes in Cp/C<sub>cr</sub> following calcium infusion as a diagnostic or prognostic guide is questionable ; and (2) the difference in the changes in serum calcium and phosphorus after calcium infusion may prove to be helpful in differentiating between uremic patients with and without marked osteitis fibrosa.