Imipenem is often used in treatment of acute exacerbations of severe chronic obstructive pulmonary disease (COPD). Superinfection following imipenem therapy is a common cause of treatment failure and high economic burden. This study is aimed to explore any clinical factors which determine the risk of superinfection after imipenem treatment in acute exacerbations of severe COPD. A prospective observational study was conducted in a 5-bed respiratory intensive care unit of a Chinese University hospital. Fifty-one patients with acute exacerbations of severe COPD who were hospitalised and treated with imipenem for more than 3 days were enrolled during 1.5 year. The associations between the risk of superinfection and potential factors were analysed by logistic regression. Forty-seven out of 51 patients (92.2%) had their symptoms and signs improved at the end of imipenem treatment. Superinfections were developed in 12 patients, and the superinfection rate was as high as 30.8% (12 out of 39 patients with definite bacteriologic responses). The frequent superinfecting organisms were Stenotrophomonas maltophilia and Pseudomonas aeruginosa. Among a wide range of potential risk factors, we found that lower blood pH, previous cephalosporines treatment and longer period of imipenem treatment are independently associated with a higher risk of superinfection. The risk of superinfection following imipenem treatment in hospitalised patients with acute exacerbations of COPD was high. Lower blood pH, previous cephalosporines treatment and longer period of imipenem treatment all increased the risk of superinfection.