We assessed the diagnostic utility of measuring C-terminal immunoreactive parathyrin (i-PTH) concurrently with free (Ca(F)) or total (Ca(T)) calcium in four groups of patients: (1) 91 patients with histologicall proven primary hyperparathyroidism; (2) seven patients without histological evidence of hyperparathyroidism; (3) 70 patients with non-parathyroid malignancies with hypercalcemia; and (4) 28 similar patients without hypercalcemia. In patients in Group 1, the use of either calcium measurement along with i-PTH increased diagnostic accuracy from 95 to 99%. In Group 2, values consistent with primary hyperparathyroidism were obtained for 21% of samples with i-PTH assay alone, 29% with i-PTH and Ca(T), and 12% with i-PTH and Ca(F). Surprisingly, 36% of samples from both malignancy groups (3 and 4) have above-normal i-PTH values. In patients with malignancy and hypercalcemia (Group 3) the following percentages were classified as possibly being hyperparathyroid: 26% with i-PTH assay alone; 31% with i-PTH and Ca(T), and 24% with i-PTH and Ca(F). Our data indicate that (a) about a fourth of patients with nonparathyroid malignancies also have evidence of hyperparathyroidism as judged by this (and probably all other) i-PTH methods based on C-terminal antibody specificity and that (b) measuring Ca(F) and i-PTH rather than Ca(T) and i-PTH gives a small increase in diagnostic accuracy.