All aerobic cells generate reactive oxygen species (ROS). Oxidative stress (excessive production of ROS) damages cells by lipid peroxidation and alteration of protein and nucleic acid structures.To prevent this damage, cells have an antioxidant system which includes such enzymes as catalase. Many lung diseases may involve ROS. In this study, we studied catalase activity, iron and transferrin in 43 children suffering from pneumonia, asthma or bronchopneumonia and 40 healthy controls. We found lower levels of iron and hemoglobin in cases and lower catalase in pneumonia and transferrin levels were higher in cases with bronchopneumonia and pneumonia than in controls