It was reported that patients with primary hyperparathyroidism (PHPT) are insulin resistant, and that surgical treatment may improve insulin sensitivity in this group of patients. The aim of our study was to evaluate the effect of surgical treatment on insulin sensitivity in patients with PHPT. Methods: In patients with PHPT (N=19; age: 58.15±8.38 years) insulin sensitivity was estimated using euglycemic hyperinsulinemic clamp (M value) before and 3 months after surgical treatment. Results: There was significant reduction of PTH (180.83±104.15 vs 46.11±19.45, P<0.05) and calcium serum levels (2.96±0.19 vs 2.29±0.17 mmol/l, P<0.05) after surgical treatment. We have observed low-normal insulin sensitivity before operation with significant improvement after surgical treatment (M value: 4.29±0.52 vs 8.21±1.44, P<0.01). There was no change in BMI (25.72±3.70 vs 24.93±3.33 kg/m2, P>0.05) and waist/hip ratio (0.82±0.11 vs 0.85±0.13, P>0.05) before and after operation (when the tests were performed). There were no correlations between chages (%Δ) of M index and PTH (r=0.32, P>0.05) and calcium (r=0.05, P>0.05) levels. In conclusion, surgical treatment improves insulin sensitivity in patients with PHPT. The mechanism of insulin resistance and its improvement after surgical treatment remains unclear in patients with PHPT.