Abstract: Objective To discuss the value of lactic dehydrogenase activity and isoenzmes in differentiating malignant and tuberculous effusion. Methods Lactic dehydrogenase activities and isoenzymes in serum and pleural fluid was determined simultaneously in 100 patients with malignancy and 81 patients with tubercuiosis. Results Lactic dehydrogenase levels and isoenzymes in pleural fluid between two groups did not correlate significantly (P>0.05).The ratio of lactic dehydrogenase activity in pleural fluid to sera between two groups did not correlate significantly (P>0.05). Conclusion Detecting lactic dehydrogenase levels and isoenzymes of pleural effusion might be helpless for differentiating malignant and tuberculous effusion.