It is generally accepted that for autonomous hyperparathyroidism, whether primary or tertiary, surgery is still the only suitable method of treatment available. Analysis of a series of cases treated in t his way over the past twenty years has shown that there are certain problems associated with the treatment of this disease that have not yet been solved. Even experienced surgeons may have difficulty in finding the parathyroid glands when the localization or number is abnormal. The histological differentiation between nodular hyperplastic, and adenomatous parathyroid tissue may be extremely difficult, sometimes even impossible. It is just this difference that might determine the amount of resection and the postoperative course. Furthermore, in some cases it is exceedingly difficult for the surgeon to judge whether a parathyroid is of normal size and has a normal appearance. Lastly, the etiology of primary hyperparathyroidism is still unknown. In the present work the findings in 267 patients treated surgically between 1950 and 1970 are discussed and evaluated in relation to the data in the literature. Special attention is paid to the etiology of the disease, the histological picture, and the surgical technique. The symptomatology, diagnosis, and biochemical changes will only be mentioned briefly, since the problems associated with them are so numerous that they deserve separate treatment. A thesis on these subjects is in preparation in Leiden. Because the prognosis of associated disease, especially the renal, is the more favourable the earlier the diagnosis is made, some principles are given to promote earlier consideration of the diagnosis hyperparathyroidism in cases with few symptoms. The data of 267 patients who underwent an operation between 1950 and 1970 under the probability diagnosis hyperparathyroidism are discussed. These patients were treated in the Leiden University Hospital, the Leiden Diaconess Hospital, or the Rotterdam University Hospital. In 255 of t hese cases the diagnosis was confirmed at surgery and histologically.