This prospective study shows that there were significant changes in the epidemiology and an increased severity of influenza A (H1N1) 2009 pneumonia during the first postpandemic influenza season. Hospitalized adults with influenza A (H1N1) 2009 pneumonia during the recent first post-pandemic influenza season were older and were more likely to have comorbid conditions than those in the pandemic period. They also had a longer time to antiviral administration and had more severe disease at hospital presentation, as evidenced by the higher percentages of septic shock, altered mental status, respiratory failure and values of PSI and CURB-65 scores. Admission to ICU and need for mechanical ventilation were also more frequent. Importantly, in-hospital mortality was four-fold higher in the post-pandemic period than during the pandemic.