Objective To investigate the clinical characteristics of severe patients with influenza A H1N1. Methods Fifteen prenant pneumonia patients with influenza A H1N1 were selected from November 26 to December 20,2009. Results The average age of all patients was 24 years old,with an average gestational age of 32 weeks. Leukopenia was observed in 13. 3% of IS patients,and lymphopenia in 86. 7%. Data on the ratio of CD4 cells to CD8 cells were available for 12 patients,54. 5% of whom had an abnormal CD4:CD8 ratio(< 1. 4). Ten of the 14 patients(71. 4%)had increased serum lactate dehydrogenase levels,which were above 245 U per liter. Four patients (26. 6%) had elevated creatine kinase levels at admission. 4 cases of 15 patients (26. 7%) had decreased serum potassium levels,which were below 3.5 mmol per liter. Four patients (33. 3%)had C4 levels higer than 36 g per liter,and 4 cases had C3 less than 0.75 g per liter. All 15 patients had radiologically confirmed pneumonia with bilateral patchy alveolar opacities, affecting three or four lung quadrants. Findings on chest radiographs were consistent with the acute respiratory distress syndrome in all patients requiring mechanical ventilation. 4 cases were found a small amount of pleural effusion, of which 1 case was combined a small amount of pericardial effusion. Respiratory distress requiring intubation and mechanical ventilation developed in 9 patients within the first 24 hours after admission, who were all pregnant women. Two of them in the third trimester died, and 7 cases who were timely terminated pregnancy were in stable condition. Conclusions Pandemic influenza A(HIM) may pose an increased risk of severe illness in pregnant women, and it is easy to develop rapidly into adult respiratory distress syndrome. The pregnancy immunological tolerance may be involved in the severe lung injury process of H1N1 influenza pneumonia.