Background: Pleural effusion is a common disorder. Based on the underlying pathology, the effusions are classified into transudates an exudates. Presently, Light’s criteria [3] are used to distinguish between transudates and exudates. But many pleural effusions have been misclassified as transudates or as exudates using Light’s criteria. The present study was done to evaluate the diagnostic accuracies of serum-effusion albumin gradient (SEAG) and pleural fluid protein thiols in differentiating pleural fluids exudates from transudates.