We retrospectively studied data of 24 patients with surgically proven hyperparathyroidism to determine whether biochemical function correlated with the calculated volume of parathyroid adenoma. Carboxyl-terminal parathyroid hormone (C-PTH), amino-terminal parathyroid hormone (N-PTH), nephrogenous cyclic AMP (N-CAMP), and other markers of calcium metabolism were measured with standard techniques. Tumors were divided into small ( < 1.0 cm<sup>3</sup>) or large (≧1.0 cm<sup>3</sup>) sizes. N-PTH and C-PTH measurements were increased in 10% and 30% of patients with small tumors and in 71 and 78% of patients with large tumors. Serum calcium, N-CAMP, C-PTH and N-PTH tended to be greater with large tumors, but only C-PTH was significant. We concluded that the size of parathyroid adenoma influenced biochemical measurements of its activity and that the measurements of PTH in patients with small tumors are not as sensitive as those in patients with large tumors.