Background Pandemic influenza A (H1N1) emerged rapidly in China in May 2009. Preliminary comparisons with seasonal influenza suggest that pandemic 2009 influenza A (H1N1) disproportionately affects younger ages and causes generally mild disease. To characterize disease progress, comorbidities, and treatment outcomes among consecutive severe and critically ill patients in a hospital served as a reference center for the care of patients with H1N1 in Shanghai,China.Methods A retrospective study on 62 severe and critically ill patients with 2009 influenza A (H1N1) was conducted in Shanghai Public Health Clinical Center. Demographic data, symptoms, comorbidities, disease progression, treatments,and clinical outcomes were collected for analysis.Results Sixty-two severe or critically ill patients were admitted to the hospital with confirmed 2009 influenza A (H1N1) infection. The median age of the study cohort was 40 years old with a range from 18 years to 75 years, and 67.7% were males. All patients presented with fever and respiratory symptoms. At presentation, 34 patients (54.8%) had comorbidities such as smoking (29.0%), hypertension (29.0%) and hepatitis B virus infection (9.7%). The median time from symptom onset to hospital admission was 6 days (interquartile-range 3-14 days) and 23 critically ill patients were admitted to Intensive Care Unit after admission. All the patients received neuraminidase inhibitors (oseltaminir), while 60 patients (96.7%) were treated with antibiotics, and 39 (62.9%) with corticosteroids. Twenty-three critical cases received noninvasive mechanical ventilation on the first day of admission, and 3 of them ultimately required invasive ventilation.Four death reports (6.5%) were filed within the first 14 days from the onset of critical illness with the primary causes of severe acute respiratory distress syndrome, hypoxemia, or complications, secondary infection and sepsis,pyopneumothorax and stroke.Conclusions Severe illness from 2009 influenza A (H1N1) infection in Shanghai occurred among young individuals.Critical cases were associated with severe hypoxemia, multisystem organ failure, and a requirement for mechanical ventilation. Most patients had a good prognosis.