Summary:Urinary adenosine 3′ 5′‐cyclic monophosphate (cyclic AMP) was assessed by a competitive protein binding method in 19 patients with primary hyperparathyroidism (PHPT) and in 15 control subjects. The mean value of 6.6 ± S.E. 0.64 μM per 24 hours for the patients with PHPT was higher (P<0.001) than the mean control value of 3.2 ±24μM per 24 hours. In all hypercalcaemic patients without renal insufficiency, urinary cyclic AMP excretion was more than 4 μM per 24 hours and decreased in the 15 patients investigated after parathyroidectomy. In a normocalcaemic hyperparathyroid patient and in three hyperparathyroid patients with renal insufficiency, urinary cyclic AMP was less than 4 μM p