In a retrospectively analyzed series of 441 patients operated for primary hyperparathyroidism (HPT), the prevalence of diabetes mellitus was 8.2%, which was three times higher than in the unselected age-matched population. Following parathyroid surgery, the need for antidiabetic treatment was unchanged. The insulin response to an intravenous glucose load was enhanced preoperatively [95 mU/1 +/- 41 (SD)] in twenty-six prospectively studied patients compared to postoperative (65 +/- 41 mU/1) investigations (P less than 0.01). This response was inversely correlated (r = 2, P less than 0.01) to the serum phosphate concentrations but not related to calcium or parathyroid hormone levels. Postoperatively, most HPT patients experienced a deterioration of their glucose tolerance (t 1/2 for i.v. glucose 54 +/- 12 and 64 +/- 21 min, respectively, P less than 0.05), and one-third of them had pathological values at follow-up. Despite this, neither the fasting blood glucose levels nor the values for haemoglobin A1c were significantly affected.