Abstract： Objective To evaluate the pulmonary imaging features in patients with severe or critical severe A H1N1 influenza. Methods Clinical and imaging findings of 18 cases with H1N1 pneumonia were retrospectively analyzed. These patients were divided into 2 groups including severe group (n=11) and critical group (n=7). Results Among the severe group, bilateral ill-defined nodules and patch shadows were found in 8 cases, local ill-defined patchy was shown in 3 cases, and consolidation of right inferior lung was demonstrated by CT scan in 1 case. Among the critical group, diffuse ground-glass attenuation with partial consolidation were found in bilateral lungs of 4 cases, subcutaneous emphysema was observed in 1 case. CT showed diffuse ground-glass attenuation and nodular like consolidation in bilateral inferior lungs in 1 case, and other 3 cases showed diffuse consolidation of bilateral lungs. Conclusions The radiologic findings of severe and critical severe pulmonary infections with H1N1 include ill-defined nodules and patch shadows of bilateral lung in sever patients, diffuse peribronchial ground-glass opacity and multifocal consolidation in critical severe patients. The radiologists should learn the features of H1N1 pneumonia on thoracic plain film and CT to make diagnosis in time.