Classification of pleural effusion (PE) is central to diagnosis. Traditional veterinary classification has distinguished between transudates, modified transudates and exudates. In human medicine PEs are divided into only two categories: transudates and exudates. The aim of this study was to evaluate, in 20 cats presented with PE, paired samples of serum and pleural fluid for the following parameters: Light's criteria (pleural fluid lactate dehydrogenase concentration (LDHp), pleural fluid/serum LDH ratio, pleural fluid/serum total protein ratio (TPr)), pleural fluid total protein, pleural fluid cholesterol concentration, pleural fluid/serum cholesterol ratio (CHOLr), serum-effusion cholesterol gradient (serum cholesterol minus PE cholesterol concentration (CHOLg)), PE total nucleated cells count (TNCCp) and pleural fluid glucose (GLUp). LDHp and TPr were found most reliable when distinguishing between transudates and exudates, with sensitivity of 100% and 91% and specificity of 100%, respectively. When conflict between the clinical picture and laboratory results exists, calculation of CHOLr, CHOLg and TNCCp measurement may help in the classification of the effusion. Measurement of serum albumin (in the case of a transudate) may provide additional information regarding the pathogenesis of the effusion.