Twenty patients being treated with thiazides were found among 95 subjects (21%) with hyercalcemia verified in repeated determinations in a health screening of 15,903 persons. There were 1,034 patients treated with thiazides in this total health screening. The prevalence of hypercalcemia in the patients treated with thiazides in this total health screening. The prevalence of hypercalcemia in the patients treated with thiazide (1.9%) was considerably higher than the prevalence of hypercalcemia found in the entire health-screened population (0.6%). The thiazide treatment was withdrawn in the 20 hypercalcemic subjects after an examination, and the patients were observed at intervals during a follow-up period of one year. The necks of 14 were explored during or after the follow-up period because of an initial serum calcium level greater than 3.0 mmole/liter or persistent hypercalcemia. Parathyroid adenomas were seen in all patients receiving surgery. Single adenomas predominated in surgical findings. The finding of the present high number of patients with primary hyperparathyroidism may be associated with elevated blood pressure resulting in thiazide treatment after detection.