Background:Little is known about the impact of community-acquired respiratory coinfection in patients with pandemic 2009 influenza A(H1N1) virus infection. Methods:This was a prospective, observational, multicenter study conducted in 148 Spanish ICUs. Results:Severe respiratory syndrome was present in 645 ICU patients. Coinfection occurred in 113(17.5%) of patients. Streptococcus pneumoniae (in 62 patients [54.8%]) was identified as the most prevalent bacteria. Patients with coinfection at ICU admission were older(47.5 ± 15.7 vs 43.8 ± 14.2 years, P<.05) and presented a higher APACHE (Acute Physiology and Chronic Health Evaluation) II score(16.1 ± 7.3 vs 13.3 ± 7.1, P < .05) and Sequential Organ Failure Assessment (SOFA) score(7.0 ± 3.8 vs 5.2 ± 3.5, P < .05). No differences in comorbidities were observed. Patients who had coinfection required vasopressors(63.7% vs 39.3%, P<.05) and invasive mechanical ventilation(69% vs 58.5%, P<.05) more frequently. ICU length of stay was 3 days longer in patients who had coinfection than in patients who did not(11 [interquartile range, 5-23] vs 8 [interquartile range 4-17], P = .01). Coinfection was associated with increased ICU mortality (26.2% vs 15.5%;OR, 1.94;95% CI, 1.21-3.09), but Cox regression analysis adjusted by potential confounders did not confirm a significant association between coinfection and ICU mortality. Conclusions:During the 2009 pandemics, the role played by bacterial coinfection in bringing patients to the ICU was not clear, S pneumoniae being the most common pathogen. This work provides clear evidence that bacterial coinfection is a contributor to increased consumption of health resources by critical patients infected with the virus and is the virus that causes critical illness in the vast majority of cases. ©2011 American College of Chest Physicians.