Serum parathormone (PTH) values are believed useful in diagnosing hyperparathyroidism and in evaluating patients before parathyroid operation. From 1975 through 1983, 328 adults patients with persistent hypercalcemia and normal renal function underwent cervical exploration for presumed primary hyperparathyroidism. Preoperative serum PTH values were obtained in 137 patients. Eight had negative explorations (5.8%): serum PTH values were normal in three patients and elevated in five. Serum PTH values were normal or low in 31 of the other 129 patients (false-negative rate = 24%). Of 191 patients for whom neither serum PTH nor nephrogenic cyclic adenosine-3',5'-monophosphate values did not were obtained preoperatively serum PTH values did not reduce the incidence of negative cervical explorations and, when obtained, they were misleading in one fourth of patients who benefitted from parathyroid exploration.