Background Initial cases of 2009 pandemic influenza A (H 1N1) virus infection having been reported mostly represented a mild illness.As the number of infected persons increased in our country,more severe infections were reported in the second half of last year.The first severely ill patient in our hospital was confirmed in October 26,2009.Methods During October 26,2009 and January 23,2010,totally 71 cases treated in our hospital who developed community acquired pneumonia from confirmed or clinically suspected 2009 pandemic influenza A (H1N1) were observed in this study,including both hospitalized patients and outpatients.Real-time reverse-transcriptase-polymerase-chain-reaction (RT-PCR) testing was used to confirm infection.Clinical data were collected retrospectively and prospectively.During 3-month follow up,high-resolution chest CT and lung function tests were performed.Results Of the 71 cases,median age was 41 (IQR 28 to 56) years; 63.4% were male,54.9% had at least one underlying medical condition.68 patients received antiviral treatment,and 33 received intravenous corticosteroids.36 of 71 patients developed acute respiratory distress syndrome during the illness.Of 25 patients who were treated by mechanical ventilation,14 survived with noninvasive ventilation,and 10 required invasive ventilation.9 patients died at a median of 16 (IQR 10 to 24) days after onset of illness.Two risk factors were identified to be independently associated with death analyzed by multivariate Cox regression:progressing dyspnea after resolution of fever (relative risk,5.667; 95%CI,1.312 to 24.475; P=0.020) and higher APACHE Ⅱ score on presentation (relative risk for each point,1.329; 95%CI,1.156 to 1.527; P < 0.001).At 3-month follow up of H1N1 pneumonia survivors,ground-glass opacities still existed in most patients (82%) with smaller extent,and lung function tests revealed that decreased diffusing capacity for carbon monoxide of mild degree is the most common (60%) abnormality.Conclusions We identified that progressing dyspnea after resolution of fever and higher APACHE Ⅱ score on presentation independently associated with death of pneumonia caused by 2009 pandemic A (H1N1) virus.Radiologic findings and lung function of some patients of H 1N 1 pneumonia did not recover fully at 3 months after onset of illness.