Crimean-Congo haemorrhagic fever (CCHF) is an acute disease affecting multiple organ systems and is characterized by fever and haemorrhages. The pathogenesis of CCHF has not been well described. Nitric oxide (NO) is an important regulator of a number of different biological processes and can participate in antimicrobial defence. In this study, we measured the level of NO in the serum of patients with CCHF and healthy controls to define the possible role of NO in the control of infection. Sixty-two patients with CCHF and 31 controls were included in the study. NO levels in CCHF patients and the control group were found to be a mean of 40.49 mu M (standard deviation (SD) 23.00) and 14.89 mu M (SD 7.94), respectively. NO levels were significantly higher in CCHF patients with respect to controls (p < 0.001). NO levels in the patients with non-fatal CCHF and fatal CCHF were compared and found to be a mean of 43.57 mu M (SD 22.70) and 26.23 mu M (SD 19.43), respectively; this difference was statistically significant (p=0.009). In conclusion, elevated levels of NO may play a protective role in CCHF.