We examined the incidence and clinical and economic consequences of primary hyperparathyroidism in residents of Rochester, Minn, from 1965 through 1976; 90 cases were found. From January 1, 1965, to June 31, 1974, the average annual incidence was 7.8 +/- 1.2 (mean +/- S.D.) cases per 100,000 population. However, after the introduction of routine measurement of serum calcium, the average annual incidence rose to 51.1 +/- 9.6 cases per 100,000. Even after availability of routine measurement of serum calcium, the annual incidence of primary hyperparathyroidism among persons 39 years of age or younger remained below 10 cases per 100,000. However, the annual incidence increased sharply in persons 40 or more years of age, reaching 188 cases per 100,000 among women 60 years of age and over and 92 cases per 100,000 among men 60 and over. For the last 1.5 years of the study, the average annual age-adjusted incidence of primary hyperparathyroidism was 27.7 +/- 5.8 per 100,000. The frequency of urolithiasis fell from 51 to 4 per cent (P less than 0.001), and the proportion of cases without symptoms or complications of primary hyperparathyroidism rose from 18 to 51 per cent (P less than 0.005). The median charge in 1977 for diagnosis and treatment of primary hyperparathyroidism was $1700. (N Engl J Med 302:189-193, 1980).