Because we experienced each 1 operative case of diffuse malignant mesothelioma of the pleura and pseudomesotheliomatous carcinoma of the lung discovered with pleural effusion, clinical comparison investigated both. The first case was suspected diffuse malignant mesothelioma of the pleura before operation, and we performed pleuropneumonectomy. But the pathologic diagnosis was adenocarcinoma of the lung, what is so called pseudomesotheliomatous carcinoma. The second case had inhaled asbestos and his pleural effusion revealed high concentrations of hyaluronic acid. Thoracoscopic biopsy showed malignant mesothelioma, and we performed pleuropneumonectomy. The pathologic final diagnosis was diffuse malignant mesothelioma of the pleura. In clinical differential diagnosis of diffuse malignant mesothelioma of the pleura and pseudomesotheliomatous carcinoma of the lung, history of inhalation of asbestos and concentrations of hyaluronic acid in pleural effusion are helpful. And thoracoscopic biopsy is necessary in established diagnosis.