OBJECTIVE: Crimean-Congo hemorrhagic fever (CCHF) is a tick-borne viral hemorrhagic fever. Disseminated intravascular coagulation (DIC) is an important complication of this disease, especially in severe and fatal cases. Antithrombin (AT) acts as an anticoagulant by inactivating thrombin, Factor IX, Factor X and Factor XI. We conducted this study to investigate the AT levels and their prognostic value in CCHF. MATERIALS AND METHODS: Twenty-eight confirmed CCHF patients were included in this study. Diagnosis of the disease was made by CCHF IgM and/or PCR positivity. Patients were grouped based on the severity criteria described previously. The patients with platelet counts <20 000×10(6) cell/L, white blood counts >10×10(9) cell/L, prothrombin times >60 seconds, aspartate aminotransferase levels >700 IU/L or alanine aminotransferase levels >900 IU/L were accepted as severe cases. Patients whose illnesses were self-limited and who did not require blood component replacement were accepted as mild cases, and patients who improved but required blood component replacement were accepted as moderate cases. Blood samples were obtained on the day that the patient had the lowest platelet count and before any thrombocyte replacement. The antithrombin activity was measured using a chromogenic substrate test (Diagnostica Stago STA Compact) at a research laboratory. RESULTS: Twenty-two (78.6%) of the cases were mild, 3 (10.7%) were moderate, and 3 were (10.7%) severe. The mean AT value was 101% for mild cases, 116.6 % for moderate cases, and 88% for severe cases (p>0.05). Although there were no statistically significant differences between the AT values, the mean AT activity was lower in severe CCHF cases. CONCLUSION: The AT activity may have been decreased in severe CCHF cases. Further studies with greater numbers of patients are required to determine the level of AT activity and its correlation with disease severity and the prognosis of CCHF.