Objective: SARS-CoV-2 has caused nearly 4 million confirmed cases of COVID-19 worldwide in the approximately 4 months since it emerged in Wuhan, China in December 2019. Comorbidities increase morbidity and mortality in COVID-19, and many laboratory parameters have been associated with mortality. The aim of the present study was to identify the relationship between endogenous carboxyhemoglobin (COHb) level and the clinical course and prognosis of COVID-19. Methods: The study included 48 non-smokers or ex-smokers aged 18 years or older who presented to the emergency department, were diagnosed with COVID-19 by real-time PCR analysis of nasopharyngeal swab sample, and were treated in the pulmonary diseases ward of the Atatürk University hospital after between March 24, 2020 and April 15, 2020. The patients’ laboratory parameters and demographic data were analyzed retrospectively. Results: Prothrombin time and C-reactive protein (CRP), troponin-I, and D-dimer levels decreased in COVID-19 patients during follow-up (p=0.024, p=0.001, p=0.001, p=0.001), while PaO2/FiO2 ratio and COHb increased (p=0.002, p=0.001). COHb level at admission was significantly lower in patients who developed macrophage activation syndrome (MAS), acute respiratory distress syndrome (ARDS), and those who died compared to the other patients (p=0.002, p=0.001). COHb level on day 5 of treatment was significantly higher in patients with ARDS and patients who died (p=0.001, p=0.001).Significant correlations were detected between COHb level and CRP (r=-0.425, p=0.001), ferritin (r=-0.395, p=0.001) and PaO2/FiO2 ratio (r=0.431, p=0.001). Conclusions: COHb level may be an easily accessible biomarker that guides early follow-up and treatment planning to avoid ARDS, MAS, and mortality in COVID-19.