A retrospective review of 500 patients with primary hyperparathyroidism seen from 1951 to 1975 was conducted; the effect of routine screening of calcium and phosphate levels (initiated in 1968) on the incidence and spectrum of the disease was analyzed. The majority of the patients (77%) were diagnosed in the eight-year period after routine biochemical screening was instituted. Comparing the group of patients diagnosed before the advent of biochemical screening and those diagnosed since screening was instituted, we found: (1) a small but significant increase in the number of asymptomatic patients diagnosed (from 2% to 12%); (2) no change in the incidence of related medical disorders, i.e., nephrocalcinosis and hypertension; (3) no change in the incidence of primary hyperplasia and adenoma; and (4) no change in the mean serum calcium level, the mean age at diagnosis, or the number or location of the involved parathyroid glands. Although routine calcium screening has identified significantly more cases of primary hyperparathyroidism, screening apparently does not enable diagnosis at an earlier stage.