A retrospective study of the case-reports of 263 patients explored surgically for primary hyperparathyroidism demonstrated the cause to be: an adenoma (206 cases), a primary hyperplasia (29 cases), or a cancer (3 cases), exploration was negative in 25 cases. Presenting symptoms were mainly urinary, but 15 p. 100 of patients seen during the last two years had been asymptomatic. Nine patients required emergency surgery and 29 had a primary normocalcemic hyperparathyroidism. The two most useful laboratory examinations, apart from measurement of blood calcium and phosphorus levels, were parathormone assay (elevated levels were present in 80 p. 100 of cases) and quantitative bone biopsy (positive in over 80 p. 100 of patients). The surgical approach was mainly cervical, except for repeat operations when ten sternotomies were performed with successful results in 4 cases. Immediate postoperative mortality was quite high (3 p. 100), particularly in the acute forms or those with multiple adenomas, and in patients over 70. Morbidity (hypocalcemia, recurrent nerve palsy) was increased after repeat surgery. Analysis of long-term results, particularly with respect to urinary symptoms, showed marked differences between lesions of single glands (adenoma) and hyperplasia. The most difficult problem to resolve with this surgery is the importance to attach to excision of the parathyroids when lesions are present in several glands.