Primary hyperparathyroidism (PHPT) was the most likely diagnosis in 68 non-thiazide-treated patients with hypercalcaemia detected in a health screening. The group comprised 55 females and 13 males, with a mean age of 55.0 +/- 0.7 (S.E.M.) years. On a pair basis these patients (the observation group) were compared with a series of 68 age- and sex-matched normocalcaemic subjects (the control group) selected from the health screening register. Renal calcui and reduced creatinine clearance were encountered less frequently in the observation group than in many reports of hospitalized patients with PHPT. Compared with the control group, the observation group comprised a greater number of subjects with renal calculi (usually multiple and bilateral), constipation, mental depression and reduced creatinine clearance. The ECG Q-T interval was shorter in the observation group than in the control group. No differences were found with respect to the occurrence of gastritis and/or peptic ulcer, poly-dipsia, polyuria and general muscle weakness. On the basis of this and a previous study it was concluded that at least 3% of the 15903 subjects participating in the health screening suffered from "asymptomatic" hypercalcaemia and most probably from "asymptomatic" PHPT.