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See related article on pages 281-287. In this issue of Seminars in Thoracic and Cardiovascular Surgery, von Wardenberg and colleagues present their data from Germany on 4763 patients who underwent cardiopulmonary bypass over a 5-year period, 127 of whom were treated with prone positioning for postoperative acute respiratory failure, the 2.7% incidence. Although their data are intriguing, the role for this application in postcardiopulmonary bypass patients in the timing of its implementation is a process in evolution. Prone positioning was first introduced over 40 years ago. In patients with severe acute ARDS, it has demonstrated improvements in shortand intermediate mortality when applied in a consistent fashion, as one would find in clinical trials. Our pulmonary and critical care colleagues use prone positioning on rather routine basis to treat patients with refractory hypoxemia. In brief, the principle is that with prone positioning, the ventral lung now becomes dependent and this ventilated lung zone is now preferentially perfused, and the dorsal lung that is now ante-dependent often becomes less consolidated in his improved aeration. This alternation of prone and supine positioning may improve oxygenation in those patients with refractory hypoxemia due to pulmonary ideology. This has become more broadly accepted in our medical intensive care units to improve oxygenation. The current study by Dr von Wardenberg and colleagues gives us at least 3 important takeaways: (1) the prone positioning is safe in cardiac surgery patients, (2) the timing of prone positioning is important, and (3) there is a gradation in severity of refractory hypoxemia that may need more aggressive therapy. The data are very well presented to demonstrate that prone positioning in cardiac surgery patients who are very close to that surgery is safe. This is a significant concern given recent