Background: Coronavirus disease 2019 (COVID-19) is a virus that is quickly spreading and has heterogeneous clinical features. Early identification of prognostic variables is necessary to coordinate treatment plans and accurately determine patient severity. Objectives: The aim of the current work was to evaluate the possible value of exhaled carbon monoxide (CO) as a marker of inflammation in different severity categories of hospitalized COVID-19 patients. Patients and Methods: A prospective cohort study was conducted on 39 confirmed COVID-19 nonsmoker patients who admitted to isolation unit at Zagazig University isolation hospital from March 2021 to February 2022. They were divided into two groups: Moderate COVID- 19 and severe COVID- 19. Exhaled carbon monoxide (eCO) was measured on admission (day 1) and after seven days (day 7). Results: It was revealed that there was high statistically significant difference between the studied groups regarding eCO at day one and seven (the level was significantly higher among severe group) (p≤0.001). Also, there were high significant positive correlations between eCO and CRP level in both moderate and severe groups through day one and seven (p≤0.001). Conclusion: It could be concluded that exhaled CO analysis can be viewed as a noninvasive inflammatory marker for determining the level and severity of inflammation as well as forecasting the prognosis of COVID-19 patients