The diagnosis of primary hyperparathyroidism has rested on the finding of hypercalcemia coupled with an elevated serum parathyroid hormone (PTH) level. Over 300 consecutive patients with primary hyperparathyroidism have had elevated or high-normal serum PTH levels using a specific immunoradiometric assay. Here we present a patient who proved to have surgically documented primary hyperparathyroidism in whom PTH levels were completely normal in all assays used. In the immunoradiometric assay, his normal result was unprecedentedly low (17 to 28 pg/mL; normal, 10 to 60 pg/mL) for this condition, and in a range consistent with non-PTH-dependent hypercalcemia or familial hypocalciuric hypercalcemia. This rare biochemical presentation of primary hyperparathyroidism should be considered in the differential diagnosis of hypercalcemia.