An unusual thymoma is reported in a 32-year-old woman. The tumor presented as diffuse pleural thickening with encasement of the left lung. Computed tomographic scan and pleural biopsy of this pleural-based lesion were interpreted as favoring a diffuse pleural mesothelioma. After pleuropneumonectomy, pathologic examination revealed a characteristic epithelial thymoma. However, neither mediastinal involvement nor a concurrent mediastinal mass was found at surgery or by radiographic examinations. To our knowledge, only six previous cases of thymomas have been reported as pleural-based tumors for which a mediastinal origin could be excluded (namely, primary pleural thymomas). It is emphasized that in clinical and pathologic diagnosis of a pleural-based lesion thymoma should be considered in the differential diagnosis.