The role of various parathyroid hormone (PTH) radio-immunoassays in the diagnosis of primary hyperparathyroidism (PHP) is controversial. A series of 204 patients with surgically proven PHP was studied. Serum total calcium, serum ionized calcium, amino (N)-terminal PTH and carboxyl(C)-terminal PTH were assessed in relation to the volume and weight of adenomatous or hyperplastic parathyroid tissue excised at operation. N-terminal PTH was elevated above the normal laboratory range in only 24% of patients and correlated relatively poorly with the volume of abnormal parathyroid tissue (r = 0.20, P = 0.05). C-terminal PTH was elevated above the normal range in 91% of patients and had a strong correlation with the volume of abnormal parathyroid tissue (r = 0.63, P less than 0.001). The correlation coefficients between C-terminal PTH and serum total calcium and serum ionized calcium were both 0.63 (P less than 0.001). In contrast, there was no correlation between N-terminal PTH and serum total calcium (r = -0.02), serum ionized calcium (r = -0.04) or C-terminal PTH (r = 0.09). A combination of hypercalcaemia and elevated C-terminal PTH can be regarded as strong diagnostic evidence of PHP. Furthermore, the level of C-terminal PTH can assist the surgeon by approximately predicting the amount of adenomatous or hyperplastic parathyroid tissue that may be expected at surgical exploration.