We prospectively assessed the diagnostic value of pleural fluid complement levels (total, C3, C4) in 135 patients with pleural effusion of five main etiologies, using novel markers. Complement levels correlated with pleural levels of protein, amylase, and transuded fluids. On univariate analysis, CHF-related pleural effusions were associated with significantly lower C4 levels than postsurgery or parapneumonic effusions. On multivariate analysis, pleural fluid C4 level was a significant predictor of CHF. Although the specificity, positive predictive value, and accuracy of the parameters were low in all diagnostic groups, their negative predictive value as well as the AUC ROC was high for CHF and post-LTX. We conclude that pleural fluid C4 levels can differentiate CHF-related pleural effusion from other etiologies and that normal level of C3 or C4 rule out CHF or LTX as causes of pleural effusion. Complement should be included in the assessment of pleural effusion when traditional diagnostic methods fail.