19 chronic renal failure patients underwent iliac crest bone biopsy prior to total parathyroidectomy with autotransplantation. The preoperative serum calcium concentration did not correlate with the number of osteoclasts/mm2 present on the preparathyroidectomy iliac biopsy. However, the postparathyroidectomy decrement in serum calcium (mg/dl and percent change) and the osteoclasts/mm2 were strongly correlated (p less than 0.001). In addition, the postoperative fall in serum calcium also correlated with the postoperative change in serum alkaline phosphatase (p less than 0.001). The nadir in postparathyroidectomy serum calcium was attained in a mean of 4.4 +/- 2.7 days. Our results indicate that the preoperative serum calcium concentration does not necessarily reflect active bone resorption, but the postoperative decrement in serum calcium provides an accurate index of preoperative histologic activity. The available data do not provide information with respect to the mechanism of postparathyroidectomy hypocalcemia since either the cessation of bone resorption, continued bone deposition, or a combination of both may be operative.