BACKGROUND: Measuring and monitoring end-tidal carbon dioxide (P-ETCO2) is an important aspect of caring for critically ill patients. The 2 methods used for P-ETCO2 measurement are the mainstream and sidestream methods. OBJECTIVE: To assess the agreement between P-ETCO2 measurements performed by mainstream and sidestream methods with the P-aCO2 values. METHODS: This was a prospective observational study. A total of 114 subjects were enrolled in the study. P-ETCO2 measurements using mainstream and sidestream methods were performed simultaneously with the arterial blood sampling in subjects who were observed in the emergency department and required arterial blood gas analysis. Agreement between the P-ETCO2 measurements and the P-aCO2 values obtained from arterial blood gas analysis were evaluated using the Bland-Altman method. RESULTS: Sixty subjects (52.6%) were female, and the mean age was 60.9 years (95% CI 58.3-63.6). The mean P-aCO2 was 35.16 mm Hg (95% CI 33.81-36.51), the mainstream P-ETCO2 was 22.11 (95% CI 21.05-23.18), and the sidestream P-ETCO2 was 25.48 (95% CI 24.22-26.75). Bland-Altman analysis showed an average difference between mainstream P-ETCO2 and P-aCO2, values of 13 mm Hg (95% limits of agreement -0.6 to 25.5) and moderate correlation (r = 0.55, P < .001). The average difference between the sidestream P-ETCO2 and P-aCO2 values was 9.7 mm Hg (95% limits of agreement -5.4 to 24.7) and poor correlation (r = 0.41, P <.001). CONCLUSIONS: P-ETCO2 values obtained by mainstream and sidestream methods were found to be significantly lower than the P-aCO2 values. There was essentially no agreement between the measurements obtained by 2 different methods and the P-aCO2 values.