The differentiation of pleural effusions as being either transudate or exudate is the first step in the diagnosis of pleural effusions. The aim of this study was to compare the efficiency of the various biochemical parameters to the traditional criteria of Light et al., for differentiating exudates from transudates. Ninety-three pleural fluid and sera specimens were obtained and classified as transudates or exudates on the basis of their diagnosis. Of the 93 pleural fluids, 21 were transudates, 72 were exudates. The efficiencies of different parameters for detection of exudates were as follows: The criteria of Light 96%; effusion cholesterol concentration 77%; serum-fluid albumin gradient 67%, pleural/serum alkaline phosphatase ratio 83%; effusion creatine kinase levels 91%; pleural/serum creatine kinase ratio 83%, and effusion uric acid 71%. Pleural/serum uric acid ratio was insignificant for the purpose of this study. (C) 1997 Elsevier Science B.V.