Primary hyperparathyroidism is most frequently caused by a single parathyroid adenoma and is more frequent in females than in males. Its frequency increases with age. Its annual incidence has been estimated at 41 per 100,000 in Rochester, and its prevalence at 4.3 per 1000 in Stockholm. Since the introduction of automated serum calcium measurements, the recognition of primary hyperparathyroidism has increased and the clinical presentation of the disease has changed. Classical osseous and renal features of the disease have become rare, due to the increased proportion of asymptomatic forms and to earlier diagnosis. Primary hyperparathyroidism is nowadays usually recognized because serum calcium is measured either systematically or in the clinical setting of mild manifestations, such as arthenia, anxiety, hypertension... Bone histology gives evidences of an accelerated bone turn-over, without major unbalance between formation and resorption. Bone densitometry has shown only minor decrease of mineral content of the lumbar spine in several modern series, so that the deleterious effect of mild or asymptomatic hyperparathyroidism on cancelous bone is presently questioned by several authors.