The prevalence of hypergastrinemia was determined in 38 consecutive patients with proved primary hyperparathyroidism. Uncorrected serum calcium levels ranged from 2.6 to 4.0 mmol/L and parathyroid hormone levels from 260 to 8750 ng/L (normal less than 600 ng/L). Preoperative serum gastrin levels were grossly elevated (1000 to 4000 ng/L) in three patients (normal median 63 ng/L; range 30 to 120 ng/L). Two patients were achlorhydric. After parathyroidectomy (adenomatous hyperplasia) in the third patient, the serum gastrin level decreased from 4000 to 3000 ng/L, with a negative response to both a secretin challenge and a meal test. The latter patient was subsequently shown to have an adrenal ganglioneuroma and islet cell hyperplasia, neither containing gastrin, and at 4-year follow-up she still has no symptoms from the hypergastrinemia. Eight patients had a modest hypergastrinemia. Serum gastrin levels returned to normal in three of the four patients after parathyroidectomy. The fourth patient had rheumatoid arthritis, which can be associated with hypergastrinemia. The mean plasma gastrin level before operation (100.3 +/- 26.1 ng/L) was similar to the postoperative value (67.0 +/- 18.5 ng/L). There was no correlation between parathyroid hormone and gastrin levels nor between serum calcium and gastrin levels. The three patients with duodenal ulcers did not have elevated gastrin levels. Therefore it would appear that routine screening of patients with primary hyperparathyroidism adds little to their clinical management.