The aim of this study was to investigate the diagnostic value of superoxide dismutase (SOD) in tuberculous pleural effusions (TPEs) and malignant pleural effusions (MPEs). Pleural effusion (PE) samples from 100 patients were classified on the basis of diagnosis as TPE (n=57) and MPE (n=43). The activity of SOD was determined by pyrolgallol assay. A significant difference was observed in SOD activity (P<0.01) between TPE and MPE, levels of being significantly higher in TPE compared to MPE. With a threshold value of 41 U/L, the area under the ROC curve was 0.653, SOD had a sensitivity of 61.4% and a specificity of 61.0% for differential diagnosis. Thus, SOD activity in PE was not a good biomarker in differentiating TPE and MPE. To the best of our knowledge, five SOD isoforms may be present in PE. Identification of which SOD contributes to the difference of SOD level between TPE and MPE is very important for illustrating mechanisms and improving the differential diagnostic value.