BACKGROUND/AIM The aim of this study is to identify the copeptin levels in patients presenting with carbon monoxide (CO) poisoning to the emergency department and to investigate its correlation with the neurological effects. MATERIALS AND METHODS The study group consisted of patients presenting with CO poisoning and carboxyhemoglobin levels >10%. Blood samples for copeptin levels were obtained twice, first at presentation then at the fourth hour of observation. The data were analyzed using SPSS 16 for Windows. RESULTS The median copeptin levels of the patient group were identified as 0.63 (0.39-1.06) ng/mL at hour 0 and 0.41 (0.31-0.49) at hour 4. The copeptin levels of the control group were 0.34 (0.25-0.42) ng/mL and were significantly lower than those of the patient group (P < 0.000). According to our results, 0.345 ng/mL for plasma copeptin level as the best cut-off level may be used with sensitivity of 94.0% and specificity of 60%. The copeptin levels at hour 0 were statistically significantly higher in the neurologically affected patients than those not affected (P < 0.001). CONCLUSION In this study it was shown that blood copeptin levels increase in patients presenting to the emergency department with CO poisoning.