Purpose: The aim of this study was to describe changes in pulmonary function from the supine to the sitting position following cardiac surgery. Methods: In a descriptive before–after design, we used the Beacon Caresystem for measurement of alveolar ventilation (V A). The Spiropalm 6MWT was used to measure tidal volume (Vt), inspiratory capacity (IC) and peripheral oxygen saturation (SpO 2). Assessments were performed 2–3 days post-operatively with patients in the supine position and again with patients sitting in a chair. Results: Fifteen patients were included. Compared to the supine position, sitting in a chair significantly improved V A by 20% (p <.000); Vt by 25% (p =.015); IC by 21% (p =.036) and SpO 2 by 0.9% (p =.018). Conclusion: Lung function including alveolar ventilation was improved in the sitting compared to the supine position and was accompanied by improved oxygenation after uncomplicated, elective cardiac surgery. Further research is warranted to describe the impact of changes in positions in patients who do not follow a standardised clinical pathway.