The first step in the diagnostic study of a pleural effusion is to classify as a transudate or exudate. Light's criteria (pleural fluid/serum proteines > 0.5; lactatedehydrogenase [LDH] > 2/3 of the upper normal limit in serum; pleural fluid/serum LDH > 0.6) usually used, incorrectly classify some cases, especially transudates. For this reason, different alternative criteria has been proposed: pleural fluid cholesterol, pleural fluid/serum cholesterol ratio, pleural fluid/serum bilirubin ratio, and serum/pleura albumin ratio. Althought the first results suggested better results that those obtained with the Light's criteria, after the analysis of the different studys we conclude that a method to diferentiate perfectly transudates and exudates is not yet available.