Urinary cyclic 3′,5′-adenosine monophosphate (cyclic AMP) was determined in two normocalcemic control groups and five groups of hypercalcemic patients. Immunoreactive parathyroid hormone was also measured by immunoassay in many of the patients. Eighteen healthy controls excreted 6.2 ± 0.5 μmoles of cyclic AMP per 24 hr; 28% excreted greater than 7.0 μmoles. Sixteen surgically proven hyperparathyroid patients excreted 7.3 ± 0.7 μmoles/24 hr, not statistically different from normal, but 73% of these patients excreted more than 7 μmoles/ hrs. Urinary cyclic AMP excretion usually decreased in these patients after surgical correction of their hyperparathyroidism. A group of four patients with suspected hyperparathyroidism excreted 5.6 ± 1.2 μmoles/24 hr, no different from either normals or surgically proven hyperparathyroid patients. Plasma parathyroid hormone was inappropriately increased for the serum calcium in all of the patients with proven or suspected hyperparathyroidism in whom it was measured. The mean urinary cyclic AMP in ten patients with hypercalcemia of non-parathyroid etiology was 3.0 ± 0.4 μmoles/24 hrs, significantly (p < 0.01) reduced from both normal and hyperparathyroid patients. Only two (20%) of these patients excreted greater than 4.0 μmoles/24 hr. This decrease was not due to wasting disease as eleven normocalcemic patients with cancer excreted 4.7 ± 0.5 nmoles/24 hr, significantly greater than the non-parathyroid hypercalcemic patients. Eight patients with probably non-parathyroid hypercalcemia excreted 3.2 ± 0.6 μmoles/ 24 hr. All four of the non-parathyroid hypercalcemia patients so tested had undetectable parathyroid hormone in the plasma. Three patients with chlorthiazide-induced hypercalcemia and ten with hypercalcemia and azotemia were also studied. Urinary cyclic AMP was nondiagnostic in the former and markedly decreased in the latter. Although parathyroid hormone determinations will probably prove to be the most accurate form of laboratory study in hypercalcemia, urinary cyclic AMP determinations as demonstrated in this study are an extremely useful tool in the differential diagnosis of hypercalcemia.