Hypercalcemia after renal transplantation (post-T hypercalcemia) has been detected in 29 (16.7%) of 174 long-term survivors. The mean time of onset of hypercalcemia was 69 days after renal transplantation (range 3-210). In 18 patients the hypercalcemia was mild and resolved spontaneously (transient) from 2-65 months (mean 19) after onset. In 4 patients serum calcium normalized concurrently with rejection episodes. In 7 patients the hypercalcemia was more pronounced (permanent), being terminated by subtotal parathyroidectomy in 5 and persisting in 2 recipients. The hypercalcemia was asymptomatic except in one patient, who developed calculi in the graft and a fall in graft function, all of which disappeared after parathyroidectomy. At operation the parathyroid glands showed hyperplasia, except in one case with an adenoma in one of the hyperplastic glands. Serum phosphorus was markedly decreased, to the same extent in transiently and permanently hypercalcemic recipients. Serum parathyroid hormone (S-PTH) was increased in all of 5 patients with permanent and in 3 of 8 with transient post-T hypercalcemia. In normocalcemic and in transiently hypercalcemic recipients the mean S-PTH was identical, but significantly lower than in the permanently hypercalcemic recipients. S-PTH was suppressed to the same extent during an i.v. calcium infusion in patients with post-T hypercalcemia and with primary hyperparathyroidism.