PURPOSE: To evaluate the differential CT features found among malignant mesothelioma and pleural metastasis from lung cancer and from extrathoracic primary tumor which on CT mimic malignant mesothelioma. MATERIALS AND METHODS: Forty-four patients who on chest CT cans showed pleural thickening suggesting malignant pleural disease and in whom this condition was pathologically confirmed were included in this study. On the basis of their pathologically proven primary disease [malignant mesothelioma (n=14), pleural metastasis of lung cancer (n=18), extrathoracic primary tumor (n=12)]. they were divided into three groups. Cases of lung which on CT showed a primary lung nodule or endobronchial mass with pleural lesion, or manifested only pleural effusion, were excluded. The following eight CT features were retrospectively analyzed: 1) configuration of pleural lesion (type I, single or multiple separate nodules, type II, localized flat pleural thickening, type III, diffuse flat pleural thickening; type IV, type III with pleural nodules superimposed; type V, mass filling the hemithorax), 2) the presence of pleural effusion, 3) chest wall or rib invasion, 4) the involvement of a major fissure, 5) extrapleural fat proliferation, 6) calcified plaque, 7) metastatic lymph nodes, 8) metastatic lung nodules. RESULTS: In malignant mesothelioma, type IV (8/14) or II (4/14) pleural thickening was relatively frequent. Pleural metastasis of lung cancer favored type IV (8/18) or I (6/18) pleural thickening, while pleural metastasis from extrathoracic primary tumor showed a variable thickening configuration, except type V. Pleural metastasis from lung cancer and extrapleural primary tumor more frequently showed type I configuration than did malignant mesothelioma, and there were significant differences among the three groups. Fissural involvement, on the other hand, was significantly more frequent in malignant mesothelioma than in pleural metastasis from lung cancer or extrapleural primary tumor . Metastatic lymph nodes and metastatic lung nodules were significantly more frequent in pleural metastasis from lung cancer and extrapleural primary tumor than in malignant mesothelioma. CONCLUSION: Malignant mesothelioma showed significantly frequent fissural involvement and the frequency with which pleural metastasis from both lung cancer and extrathoracic primary tumor showed type I pleural lesion, metastatic lymph nodes or metastatic lung nodules, was significantly frequent. Even though no CT features for differentiating between pleural metastasis from lung cancer and from extrathoracic primary tumor were found, the CT features stated above would help differentiate malignant mesothelioma from the other two groups.