The increase of nephrogenic cyclic AMP is an excellent index of parathyroid hypersecretion. A successful treatment of primary hyperparathyroidism results in a rapid fall in nephrogenic cAMP. In a series of 24 patients with proven primary hyperparathyroidism (hyperplasia 3, adenoma 21) and 2 patients with suspected hyperparathyroidism, the success of surgical excision was evaluated by measuring the urinary cAMP/urinary creatinine ratio (R), which in the absence of renal impairment, is proportional to the level of nephrogenic cAMP. Sequential assays of urinary cAMP and creatinine were performed during surgery; laboratory results were available within less than one hour. Among 22 patients with elevated baseline value or R, R became normal in 18 and decreased by more than 50% in 3; these findings suggested that the operation would be successful. In 1 case, R was not measured as the patient had impaired renal function. In another patient with normal baseline value of R, R did not significantly decrease after excision. Surgery failed in 1 patient, although the high value of R at the end of the operation should have prompted us to continue. Finally, in 2 patients the diagnosis was erroneous since R was lower than 0.5 as in controls. Surgeons, therefore, now have a reliable biochemical method at their disposal, but its use will be limited by its cost and complexity.