Summary. From i960 through April t974 in the Department of Surgery of University inMunic 90 patients were operated in suspicion of a primary hyperparathyreoidism. In 73 cases the diagnosis was verified. The review allows following conclusions:1. Essential for surgical exploration is a nearly certain preoperative diagnosis. The calcium infusion test and the iliac crest biopsy are approved methods to clarify the diagnosis. Both methods have been positively in 89 % of the cases in its own combination.2. The diagnosis and localisation of morphological change of parathyroid glands are not possible. Therefore one should always attempt to identify all parathyroid glands and to carry out a biopsy for establishing a definitive diagnosis. To avoid reoperations in case of a primary hyperplasia a subtotal parathyroidectomy is necessary.3. An improvment of diagnosis of primary hyperparathyroidism is expected if the radioimmunoassay of PTH is available in all clinics.