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Introduction: Volumetric capnogram is composed of three phases. The CO2 slope is part of the capnogram and the slope represents Phase III of a volumetric capnogram. According to previous studies, the CO2 slope is associated with respiratory system resistance in patients with normal pulmonary function and in critically ill patients. However, the relationship between CO2 slope and other lung mechanics (compliance, time constant and time constant for exhalation) measured and displayed by the ventilator, is not well known. Hypothesis: It is hypothesized that CO2 slope is associated with lung mechanics that are measured and displayed by some ventilators. Methods: Patients receiving mechanical ventilation were consecutively recruited to study the association of CO2 slope and lung mechanics using the Evita Infinity V500®. Patients with spontaneous breathing were excluded. Results: Ten mechanically ventilated patients, without spontaneous breathing, were enrolled in the study. Eight patients were male; the mean age was 58.9 ± 23 (SD). Regression analysis found a good correlation between CO2 slope and respiratory system resistance (r=0.90, p<0.001), a weak correlation between CO2 slope and compliance (r=-0.58, p>0.05), a weak correlation between CO2 slope and time constant for exhalation (r=-0.28, p>0.05), and no correlation between CO2 slope and time constant (r=0.06, p>0.05). Conclusions: On mechanically ventilated patients, without spontaneous breathing, CO2 slope is significantly correlated with respiratory system resistance and not significantly correlated with compliance, time constant or time constant for exhalation.