Glucose tolerance, insulin secretion, and insulin sensitivity were evaluated in 8 asymptomatic patients with primary hyperparathyroidism (PHPT) before and at least 8 weeks after surgical correction of PHPT by means of the hyperglycemic clamp technique. In addition, 15 sex- and agematched control subjects were investigated for comparative reasons by the same technique. Glucose metabolized (M) during the hyperglycemic clamp was not significantly (NS) different between patients with PHPT and controls (7.9±2.3 vs. 6.3±1.9 mg/kg/min). However, insulin secretion (I) was significantly elevated in patients with PHPT compared to controls (87±17 vs. 45±12 μU/ml,P<0.05). The calculated insulin sensitivity index, (M/I) was significantly reduced in PHPT compared to controls (11.0±2.1 vs. 15.2±1.4 mg/kg/min per μU/ml×100,P<0.05). Comparing patients with PHPT before and after surgery, the M value, which is a measure of glucose tolerance, was not significantly different (7.9±2.3 vs. 7.8±1.5 mg/kg/min). However, insulin secretion was significantly lower after surgical correction of PHPT compared to the preoperative situation (48±9 μU/ml vs. 87±17 μU/7 ml,P<0.01). The calculated M/I rose significantly after surgery compared to the preoperative value (11±2.1 vs. 17.6±2.7 mg/kg/min per μU/ml ×100,P<0.001). We conclude that disturbed carbohydrate metabolism such as insulin hypersecretion and insulin resistance, in patients with PHPT is an early finding in this disease and that these early disturbances in glucose metabolism are, however, fully reversible. Correction of disturbed carbohydrate metabolism in PHPT might be a distinct argument for early surgical intervention in this disease.