In January and February 2011, we experienced on 3 cases of severe pneumonia caused by influenza A (H1N1) 2009 pdm. The patients were 2 males and 1 female: 2 were obese, 2 with psychiatric disorders, 1 was diabetic, and 1 had been vaccinated for the influenza virus. The disease progressed in the 5 to 7 days from the onset of symptoms to admission with acute respiratory failure. Although nasal swab rapid-diagnostic tests were negative except for one patient, the final diagnosis of the other pneumonia was later made. A detection of the other RNA in two cases and with elevation of anti-H1 antibody titer by paired serum in the other. HRCT images demonstrated panlobular ground glass opacities in all cases. A transbronchial lung biopsy, performed in two cases, detected diffuse alveolar damage in one case and organizing pneumonia in the other. As for treatment, along with oseltamivir and/or peramivir applied as antiviral drugs, steroid pulse therapy and/or polymyxin-B direct hemoperfusion (PMX-DHP) therapy were applied in two cases. One patient was intubated and mechanically ventilated, and the other was noninvasively ventilated. All cases were fully recovered and discharged, although we experienced difficulty in initial diagnosis and treatment.