This is a report of 615 patients who were operated on for primary hyperparathyroidism during the 20 years from 1950 to 1979. There were twice as many females in the series as males, mainly because of a sharp rise in the incidence in females over the age of 45 years. Identification of all parathyroid glands and confirmation by frozen section was always attempted. Only macroscopically enlarged glands were removed, regardless of histologic findings. Enlargement of I gland was found in 433 patients, of 2 glands in 106, and of 3 glands in 43. In 33 patients all glands were enlarged. Bilateral exploration of the neck is mandatory in order to prevent persistent hyperparathyroidism. An abnormal location of parathyroid glands was found in 263 patients; location within the thymus was the most frequent. Most ectopically located parathyroids can be removed through the neck. Sternotomy gave disappointing results; in only 3 of 9 patients was an enlarged parathyroid found. Persistent hyperparathyroidism necessitating reoperation occurred in 25 patients. The results of reoperation were not as favorable as those of primary operations and gave rise to more complications, especially inadvertent removal of all parathyroid tissue. The mean follow-up time was 5.6 years. During this period no patient developed true recurrent hyperparathyroidism arising in a previously identified normal appearing gland. Therefore, we advise removal of only those glands that are macroscopically enlarged.