Serum immunoreactive PTH (iPTH) was measured in 74 patients with urolithiasis and correlated to the corresponding serum calcium values. Serum iPTH was measured using a rooster antibovine iPTH antiserum which crossreacted with the human hormone within the 44-68 amino acid residue region. Sixty-six of these patients had normal serum iPTH and calcium concentrations. Their calcium values varied from 2.2 mmol/l to 2.6 mmol/l and their serum iPTH concentrations were less than 0.6 micrograms/l. The remaining 8 patients with urolithiasis were judged to have primary hyperparathyroidism because of an abnormal iPTH/calcium relationship. These patients had serum calcium concentrations varying from 2.6 mmol/l to 3.4 mmol/l and iPTH concentrations between 0.35 micrograms/l and 3.03 micrograms/l. The diagnosis was verified histologically in 7 patients after operation. In the last patient iPTH was reduced from 1.01 micrograms/l to 0.21 micrograms/l after surgery, and serum calcium changed from 2.6 mmol/l to 2.2 mmol/l. The combined evaluation of serum iPTH and calcium may improve the diagnosis for hyperparathyroidism and was in our series helpful in making a correct diagnosis in 2 out of 7 patients who had histologically verified disease. In addition, iPTH measurements are valuable to rule out hyperparathyroidism as the cause of hypercalcaemia.