The secretion of intact parathyroid hormone (PTH) was investigated in 11 patients operated on for parathyroid adenoma at 1 year after surgery and compared with that of seven healthy individuals and five patients operated on because of clinical and biochemical signs of primary hyperparathyroidism with equivocal diagnosis after surgery. The investigation was performed by infusing Na2EDTA and CaCl2 at constant rates. No significant difference was found in the suppressibility of PTH secretion by calcium. The set point (the calcium concentration required for half-maximal inhibition of PTH secretion) was slightly lower in patients (1.20 +/- 0.02 mmol/l) compared with healthy subjects (1.23 +/- 0.03 mmol/l; P < 0.05). During the hypocalcemic EDTA infusion, the secretion of PTH was higher in controls compared with patients (P < 0.01). By comparing the data from the infusion tests in patients operated on for parathyroid adenomas with the data obtained from the patients with equivocal diagnosis after parathyroid surgery, a good probability for the diagnosis could be obtained.