Background: Crimean-Congo hemorrhagic fever (CCHF) is a systemic viral disease that also affects the endothelium. Thrombocytopenia and hemorrhage are seen in this disease. But, the cause of thrombocytopenia is not clear. We hypothesized that endothelium dysfunction may be the cause of thrombocytopenia. We evaluated the endothelium functions by flow-mediated dilatation (FMD) in CCHF.