Whether fluid accumulating in the pleural space is a transudate or an exudate is determined by the widely used criteria of the pleural fluid to serum LDH and protein concentration ratios. Such a distinction is important for limiting the extent of the differential diagnosis of possible causes for this condition. We have found that a pleural fluid to serum total bilirubin ratio can serve the same purpose. The correlation of a bilirubin concentration ratio of 0.6 or more with the presence of an exudate as determined by established criteria is statistically highly significant; and its sensitivity, specificity, positive predictive accuracy, and overall accuracy in relation to etiology and LDH or protein criteria (Light's criteria) are about 90 percent. Hence, the bilirubin criterion is statistically equivalent to the widely accepted LDH and protein criteria.