Ionized and total Ca concentrations were measured in the same serum samples of 48 patients with primary hyperparathyroidism in order to compare diagnostic accuracy and sensitivity and to determine the intra-individual variation of these parameters in this disorder. In 149 samples, 0.7% of serum ionized Ca and 7.4% of total Ca determinations were within the normal range (mean .+-. 2 SD). All patients exhibited abnormally elevated values upon repeated testing. Ionized and total Ca were significantly correlated both preoperatively (r = 0.85 and P < 0.001) and postoperatively (r > 0.9). Heterogeneous and abnormally large variations of both measurements were noted in these patients. The superior precision of the method for ionized Ca determination renders it more accurate than total Ca for the diagnosis of primary hyperparathyroidism. The disturbance of Ca homeostasis is reflected in both total Ca and its ionic fraction in serum.