objective: crimean-congo hemorrhagic fever is an acute viral hemorrhagic fever with a high mortality rate. despite increasing knowledge about hemorrhagic fever viruses, little is known about the pathogenesis of crimean-congo hemorrhagic fever. in this study, we measured serum adenosine deaminase and xanthine oxidase levels in crimean-congo hemorrhagic fever patients. methods: serum adenosine deaminase levels were measured with a sensitive colorimetric method described by giusti and xanthine oxidase levels by the method of worthington in 30 consecutive hospitalized patients (mean age 42.6 ± 21.0). laboratory tests confirmed their diagnoses of crimean-congo hemorrhagic fever. thirty-five subjects (mean age 42.9 ± 19.1) served as the control group. results: there was a significant difference in adenosine deaminase and xanthine oxidase levels between cases and controls (p<0.05). however, neither adenosine deaminase nor xanthine oxidase levels varied with the severity of disease in the cases assessed (p>0.05). conclusion: adenosine deaminase and xanthine oxidase levels were increased in patients with crimean-congo hemorrhagic fever. elevated serum xanthine oxidase activity in patients with crimean-congo hemorrhagic fever may be associated with reactive oxygen species generated by the xanthine/xanthine oxidase system during inflammatory responses. in addition, elevated lipid peroxidation may contribute to cell damage and hemorrhage. the association of cell damage and hemorrhage with xanthine oxidase activity should be further investigated in large-scale studies.