Purpose: Carbon monoxide (CO) poisoning is one of the most common causes of fatal suicide or accidental poisoning worldwide. There is no specific biomarker in the diagnosis. Therefore, the role of whole blood parameters in CO poisoning was investigated. Material and Methods: A total of 442 patients diagnosed with CO intoxication at the emergency department between January 1, 2014 and January 1, 2016 were included in the study retrospectively. The demographic characteristics of the patient group and the complete blood results were obtained from the hospital registry system. The control group consisted of 100 patients presenting with similar complaints to the emergency department. Leukocyte (WBC), red cell distribution width (RDW), platelet, lymphocyte, neutrophil, neutrophil lymphocyte ratio (NLR) and platelet lymphocyte ratio (PLR) values of both groups were compared statistically. Results: The patient and control groups were similar in age and gender. The WBC, RDW, Platelet, Lymphocyte, Neutrophil, NLR and PLR levels of the patient group were significantly higher compared to the control group. However, there was no significant correlation between patient admission time and Glaskow Coma Scale (GCS), carboxyhemoglobin (COHb), RDW, NLR and PLR levels. Conclusion: Our study suggested that RDW, NLR and PLR levels have potential to be a useful marker for the diagnosis of CO poisoning.