Idiopathic pulmonary fibrosis (IPF) and chronic obstructive pulmonary disease (COPD) are thought to involve lung injury induced by reactive oxygen species (ROS), in particular superoxide anion. The enzyme, superoxide dismutase (SOD) catalyses the dismutation of superoxide anion to hydrogen peroxide. Lecithinized SOD (PC-SOD) has overcome a number of previous clinical limitations of SOD, including low tissue affinity and low stability in plasma. Recent animal studies suggest that PC-SOD is effective for the treatment of IPF and COPD. We are now performing the clinical study of PC-SOD for IPF patients.