Parathyroid adenoma is responsible for 80-85% of cases of primary hyperparathyroidism. Increased fibrinogen levels in patients with adenoma may increase the risk of atherosclerosis and cardiovascular events. The aim of this study was to investigate the relationship between coagulation parameters and parathyroid adenoma. A prospective study included 28 female patients with parathyroid adenoma aged 40-88 years and 27 age-matched healthy controls. The coagulation parameters were assessed for each participant. The mean ages of the patient and control groups were 57.7 +/- 10.9 and 53.3 +/- 9.31 years, respectively. The mean level of protein S activity was 65.79 +/- 13.78 in the patient group and 77.00 +/- 15.72 in the control group, and the difference was statistically significant (p = 0.013). The mean fibrinogen levels of the patient and control groups were 338.78 +/- 63.87 mg/dL and 304.30 +/- 45.67 mg/dL, respectively, and a significant difference was found (p = 0.041). However, no significant difference was evident between the two groups with regard to the D-dimer (p = 0.238), aPTT (p = 0.645), INR (p = 0.406), protein C (p = 0.076), and AT-III (p = 0.180) levels. A positive correlation was observed between adenoma volume and fibrinogen in the patient group (r = 0.711, p = 0.001). The protein S levels were lower and the fibrinogen levels higher in the patients with parathyroid adenoma.