Of 570 patients operated on for hyperparathyroidism, 18 subsequently died (between one day and 12 yr after operation). Pathological findings at necropsy in these patients have been reviewed. Four necropsies on patients with untreated hyperparathyroidism have also been reviewed for comparison. Of these 22 cases, 14 had renal failure at the time of death. This was attributed to the following: nephrocalcinosis 6; chronic glomerulonephritis 4; analgesic nephropathy 1; cystinuria 1; sarcoidosis 1; and polycystic disease 1. Of those with renal failure due to nephrocalcinosis, three had persistent hypercalcaemia after operation; the other three survived only for a short time. There was no evidence of residual nephrocalcinosis producing progressive renal failure if the plasma calcium concentration was corrected. In those with chronic glomerulonephritis, cystinuria, and polycystic disease, the hyperparathyroidism was considered to be tertiary to the renal disease but renal failure was not always present at the time of diagnosis.