Endocrine abnormalities in 73 patients with primary hyperparathyroidism were studied: 8 patients were diabetic, 3 were thyrotoxic, 2 were hypothyroid, and one had a gastrinoma and carcinoid tumour. In 9 of these patients, carbohydrate metabolism and thyroid function were studied in detail pre- and 3 months postoperatively. In 6 patients following a glucose load, insulin concentrations were markedly elevated pre-operatively when compared with those found postoperatively. Likewise, in 7 patients, serum thyroxine was greater pre- than postoperatively. The aetiology of these changes and their relevance to the management of patients with primary hyperparathyroidism is discussed.