Nineteen consecutive patients receiving renal transplants underwent prospective evaluation of their calcium homeostasis for 1 year after transplantation to characterize indices of hyperparathyroidism (HPT) amelioration. All but one underwent dialysis, and six had vitamin D supplementation before grafting. The rapid falls in serum creatinine concentrations and increased creatinine clearances the first weeks after grafting were accompanied by rapidly reversed hypercalcemia and hypermagnesemia, induced hypophosphatemia, maintained parathyroid hormone (PTH) excess and calcitriol deficiency, and decreased alkaline phosphatases. At 3 months when the serum calcitriol had started to rise, serum PTH levels were the lowest and parathyroid responses to induced hypocalcemia the least abnormal. This was coupled to peaks in serum calcium, 24-hour urine calcium excretions, and serum alkaline phosphatase levels. All patients had subnormal creatinine clearances at the study end, and normal serum PTH occurred in only seven of them. Arbitrary subgrouping of the material was performed according to posttransplant creatinine clearance and serum PTH levels. More satisfactory graft function related to lower serum PTH values and less abnormal parathyroid responses to induced hypocalcemia, earlier and higher rises in serum calcitriol, and higher urine calcium excretion. Patients with mild HPT at the study end generally had higher creatinine clearance, lower serum PTH, calcium, and alkaline phosphatase values, and lower urine calcium excretion. Moreover, they had fewer prevalent signs of radiologic bone involvement before grafting. These temporal diversities in conjunction with the variable graft function and intensity of immunosuppression provide a complex interaction in renal transplant recipients, which should be considered in the light of improved function of the PTH/PTHrP receptor in bone and kidney and cation receptors in the parathyroid and kidney.