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We performed an observational analysis of a prospective cohort of adults hospitalized for pandemic (H1N1) 2009 at 13 Spanish hospitals, from June to November 2009, to determine the risk factors, clinical features, and outcomes of pneumonia. Of 585 patients requiring hospitalization, chest radiography was obtained in 542. A total of 234 (43.1%) patients had pneumonia, of whom 210 underwent bacterial microbiologic studies. Of these patients, 174 (82.8%) had primary viral pneumonia and 36 (17.2%) had concomitant/secondary bacterial pneumonia. Bilateral pneumonia occurred in 48.3% of patients. Streptococcus pneumoniae was the most frequent pathogen among patients with bacterial pneumonia (26 of 36 patients). None of them had received pneumococcal vaccine. Compared with patients without pneumonia, those with pneumonia more frequently had shock during hospitalization (9.8% vs. 1%; p < 0.001), required intensive care unit admission (22.6% vs. 5.8%; p < 0.001), underwent mechanical ventilation (17.9% vs. 3.2%; p < 0.001), and had longer length of hospital stay (median, 7 d vs. 5 d; p < 0.001). In-hospital mortality was higher in patients with pneumonia than in the others (5.2% vs. 0%; p < 0.001). Absence of comorbid conditions (odds ratio [OR], 2.07; 95% confidence interval [CI], 1.32-3.24) was found to be an independent risk factor for pneumonia, whereas early (≤48 h) oseltamivir therapy (OR, 0.29; 95% CI, 0.19-0.46) was a protective factor. In conclusion, pneumonia is a frequent complication among adults hospitalized for pandemic (H1N1) 2009 and causes significant morbidity. Mortality in pandemic (H1N1) 2009 is low, but occurs mainly in patients with pneumonia. Early oseltamivir therapy is a protective factor for this complication.