The analysis of serum and body fluids for isoenzymes have been intensively applied to clinical diagnosis. The aim of the present study was to determine the role of lactic dehydrogenase (LDH) isoenzymes in the diagnosis of malignant pleural effusion and its differentiation from other effusions. The activity of total LDH in samples of serum and pleural fluids were determined simultaneously in patient groups with nonspecific pleurisy, tuberculous pleurisy (Tbc), empyema, congestive heart failure (CHF), and malignancy. In our data, the mean value of total LDH activity in empyema group was significantly higher than the others. Isoenzyme patterns of empyema group were also significantly different than the other four groups. There were marked increases in LDHi and LDHs in malignancy group, the latter was signicantly different than the other groups while LDHi was not significantly different than the group with CHF. Malignancy showed significant reduction in LDH4 and LDH5 while marked increase was seen in LDH4 and LDHs in empyema group. There was no relationship between histologie pattern and LDH isoenzyme pattern of the pleural fluid in malignancy. So the electrophoretic determination of LDH isoenzyme pattern in pleural fluid can be predictive in distinguishing malignancy from empyema. But this tool can not help us in distinguishing pleural effusions due to malignancy from other types of exudative pleural effusions.