Twenty-seven hypercalcaemic subjects were identified in three generations of a family. There were no clinical complications of chronic hypercalcaemia, but five had had parathyroid surgery which was unsuccessful in four. Twenty of the twenty-seven subjects were compared with twenty-four normocalcaemic controls from the same family and the findings were also compared with those from forty patients with surgically proven primary hyperparathyroidism. The relation between the serum and urinary calcium levels was studied by means of an oral calcium loading test. The ratio of calcium clearance to creatinine clearance was normal in this family (but elevated in the patients with primary hyperparathyroidism) and the concentration of parathyroid hormone was normal, as was the total urinary excretion of cyclic AMP. Thus, there was no evidence of either suppressed or increased parathyroid activity in this familial condition. Basal urinary calcium excretion was normal under steady-state conditions indicating that the hypercalcaemia could not be attributed to either increased bone resorption or increased calcium absorption from the gut. In accordance with this, the serum levels of 1,25-dihydroxycholecalciferol were normal. The hypercalcaemia in this condition can be accounted for in full by an increase in renal tubular reabsorption of calcium, and thus differs from that of primary hyperparathyroidism in which there is increased production of calcium from gut and/or bone as well as an increase in renal tubular reabsorption of calcium. Although the serum phosphate and renal tubular reabsorption of phosphate were both low in patients with familial benign hypercalcaemia, they were not as low as in patients with the same degree of hypercalcaemia due to primary hyperparathyroidism. The changes in phosphate transport in familial benign hypercalcaemia could be explained as a secondary effect of the increased filtered load of calcium in the kidney. The tendency towards hypermagnesaemia in our patients, which contrasts with a tendency towards hypomagnesaemia in primary hyperparathyroidism, could also be explained as a secondary effect of the abnormality of renal tubular reabsorption of calcium. Increased renal tubular calcium reabsorption and persistent normal functioning of the parathyroid glands in the face of hypercalcaemia remain the sole definite abnormalities of the syndrome.