The limited usefulness of radioimmunoassays of parathyroid hormone in the differential diagnosis of hypercalcaemia invites the use of methods measuring effects of parathyroid hormone (PTH). Data from 4-5-day metabolic studies in 107 hypercalcaemic patients (78 retrospective and 20 prospective cases) were combined in a hypercalcaemia discrimination index (HDI). HDI = (urine calcium (mg/24 h) x 100 x serum phosphate (mg/100 ml))/(serum total calcium (mg/100 ml)x 24-h clearance of creatinine (ml/min)) expresses in one figure the combined actions of PTH on the renal handling of calcium and on serum phosphate. A multivariate analysis confirmed that HDI offered optimal discrimination. An identical discrimination was observed in the prospective series. In the complete series HDI = 137 was the optimal discrimination point. Classification of the patients as having hyperparathyroidism (HPT) or pseudohyperparathyroidism (P-HPT) (<137) or non-parathyroid hypercalcaemia (NON-PTH) (≥ 137) corresponded in 100 out of 107 patients (93.5%) with the final clinical diagnosis. The effects on HDI of sex, age, season, urine losses, high calcium intake and use of thiazides were also evaluated. HDI appears to be a valuable tool in the endocrine evaluation of hypercalcaemic patients. Used in combination with radioimmunoassays measuring genuine but not ectopic PTH HDI may serve to classify hypercalcaemic patients within the following 3 subgroups: HPT, P-HPT and NON-PTH.