BACKGROUND: Tests able to help in the diagnostic work-up of pleural exudates are needed. C-reactive protein (CRP) may be useful for distinguishing between benign and malignant exudates. METHODS: A total of 123 consecutive patients diagnosed as having exudative pleural effusion (60 associated with malignancy and 63 benign effusions) were included in the study. Sensitivity, specificity, positive and negative predictive values (PV+, PV-), and positive and negative likelihood ratios (LR+, LR-) were established at different cut-off points. RESULTS: Pleural fluid CRP level was 23+/-12 mg/l (mean+/-S.D.) in pleural exudates associated with malignancy and 50+/-33 mg/l in benign effusions (P<0.001). With a cut-off point below 20 mg/l for malignancy, sensitivity of CRP was 0.50, specificity 0.89, PV+ 0.81, PV- 0.65, LR+ 4.50, and LR- 0.65. With a cut-off point above 45 mg/l for benign diseases, sensitivity was 0.44, specificity 0.95, PV+ 0.90, PV- 0.62, LR+ 8.89, and LR- 0.58. CONCLUSIONS: The pleural CRP level provides useful information for the study of pleural exudates. A level below 20 mg/l suggests a malignant origin and a level above 45 mg/l virtually rules out this possibility. Additional advantages of measuring CRP level are that it is an inexpensive test and is easy to perform.