Patients were identified from a unit-based acute lung injury screening database (Cleveland Clinic is one of the centers participating in the ARDSnetwork) and the H1N1 patient log maintained during the fall-winter season of 2009-2010. Patients were included if they met criteria for ALI (PaO 2 /FiO 2 ≤300; acute bilateral infiltrates; positive pressure ventilation via endotracheal tube; and no clinical evidence of left atrial hypertension or congestive heart failure) between the months of September 2009 to March 2010-the time that influenza infection was most prevalent. Diagnostic methods for influenza A virus detection consisted of rapid antigen testing, polymerase chain reaction (rtPCR), and viral culture from nasopharyngeal swabs, tracheal aspirates, and bronchioalveolar lavage specimens. The patients were grouped into two categories: those with laboratoryproven H1N1 infection; and those in whom H1N1was not clinically suspected. Only patients with confirmed infection were included in the influenza group to ensure that the clinical course of the disease was accurately captured. Patients were excluded from the study if they did not meet the above criteria for ARDS, or if clinical suspicion pointed to a likely pandemic viral infection with negative diagnostics.