Serum calcium was measured in 12,339 men and 13,394 women ages 25 to 75. Primary hyperparathyroidism, defined as a combination of serum calcium and parathyroid hormone (PTH) levels within the extreme or upper normal range, was diagnosed in 17 men and 47 women. The prevalence in both sexes increased with age. When 42 subjects with asymptomatic primary hyperparathyroidism were followed for 3 years, no significant increase in serum calcium or PTH was seen. In a subgroup of 473 men and 517 women ages 50 to 75, serum PTH was measured along with serum calcium. Depending on the criteria used to define primary hyperparathyroidism, the prevalence in older women within this subgroup ranged from 3.6% to 13.9%. The study concluded that a high prevalence of primary hyperparathyroidism exists in older women, although the progression of the disease, judging by serum calcium and PTH measurements, appears to be very slow.