In this study, the investigators analyzed the efficiencies of hyaluronan, osteopontin, C-ERC/mesothelin, N-ERC/mesothelin and syndecan-1 serum levels, both individually and in combination, in distinguishing malignant pleural mesothelioma patients from patients with metastatic malignant pleural diseases, benign pleural diseases, and benign asbestos pleurisy.