Demographics, presenting symptoms, past medical history, and acuity on admission are shown in Table 1 . Patients in the influenza group tended to be younger with a higher BMI. Patients in the influenza group presented more often with lower respiratory infection (100 vs. 73%, p = 0.135) and had increased requirement for mechanical ventilation on admission to the ICU (96 vs. 68%, p = 0.022). On the other hand, the noninfluenza group had a higher propensity to present with shock requiring vasopressors (45 vs. 22%, respectively, p = 0.07). The primary cause of ALI in the H1N1 group was pneumonia (n = 23), whereas in the noninfluenza group the etiologies were more varied, including pneumonia (n = 9), sepsis (n = 5), aspiration of gastric contents (n = 2), transfusion reaction (n = 1), and other (n = 5). Whereas seven patients (30%) in the H1N1 group were considered healthy, only one patient (5%) in the noninfluenza group had no comorbid medical conditions on admission to the ICU (Table 1) . This difference is reflected in the lower mean APACHE III score on admission to the ICU in the H1N1 group (66 ± 20 vs. 89 ± 32, p = 0.015), despite similar SOFA scores (8.3 ± 3.4 and 9.2 ± 4.1, p = 0.44).