The relative value of each lactic dehydrogenase isoenzyme (iso LDH) was measured by electrophoretic separation in the serum and the pleural fluid of 100 patients. In each case, the cause of the pleural effusion was known. Two types of LDH isoenzyme pattern were found in the serum: a normal type with a low value of LDH 5 and an abnormal type with a high value of LDH 5. This high LDH-5 level is due to an impaired liver function. In the pleural fluid, the electrophoretic patterns of five LDH isoenzymes were found by computerized processing. During congestive heart failure (28 cases) the electrophoretic pattern of the LDH isoenzymes was always similar in the serum and in the pleural fluid (transudative pleural effusion). During thoracic empyema, the relative values of the isoenzymes in the pleural fluid were regularly increasing from LDH 1 to LDH 5. In this situation, the evaluation of LDH 5 appeared to emanate from the increased granulocytes in the pleural fluid. In 22 inflammatory pleural effusions, the relative values of the five isoenzymes were equal. During malignant effusions (35 cases) a high level of LDH 5 was found in 21 patients. LDH 5 is known to be secreted by malignant tissue, and the authors confirmed it by finding a high level of LDH 5 in biopsy specimens of patients with mesothelioma or epidermoid lung cancer (7 cases). Conversely, the level of LDH 5 was low in biopsy specimens from normal lung tissue or benign inflammatory pleuritis (6 cases). Among the 14 patients with low levels of LDH 5 in the pleural fluid during malignant pleural effusion, the authors found the malignant lymphomas (three cases) and the small cell lung carcinoma (five cases). In these cases, the low level of LDH 5 was in agreement with the result of a low level of LDH 5 found in the biopsy of a metastatic liver localization of a small cell lung carcinoma. So the electrophoretic determination of LDH isoenzymes pattern in pleural fluid is a sensitive tool for the management of pleural effusion.