Plasma calcium and albumin levels were measured serially in 100 patients for two years following successful renal transplantation. Mean plasma calcium increased during the first six months after grafting, in large part attributable to an increase in plasma albumin. The variance around the mean plasma calcium did not increase suggesting that mechanisms responsible for hypercalcaemia were common to the majority of patients. 36 per cent of patients developed hypercalcaemia within two years of grafting but the incidence fell to 11 per cent when more rigorous criteria for hypercalcaemia were used. The mechanisms maintaining plasma calcium were studied in 29 of the patients, nine of whom were hypercalcaemic and 20 of whom were normocalcaemic. Before transplantation, mean plasma calcium and phosphate levels were higher, the prevalence of subperiosteal erosions and extraskeletal calcification radiographically was greater, and the duration of haemodialysis treatment was longer in the hypercalcaemic patients than in the normocalcaemic recipients. At assessment after transplantation, hypercalcaemic patients had lower levels of plasma phosphate, higher plasma levels of alkaline phosphatase and parathyroid hormone, and higher hydroxyproline excretion. Renal function and 47Ca absorption were similar in the two groups. The major cause for apparent hypercalcaemia in transplanted patients appeared to be an increase in plasma albumin. In patients with true hypercalcaemia the major cause was pre-existing hyperparathyroidism where hypercalcaemia was mediated by increased renal tubular reabsorption of calcium.