Abstract We described 2 cases of plasmacytoma presenting with a preponderant involvement of the pleural membranes simulating clinically, radiologically, and on gross pathologic inspection a primary mesothelioma. The patients were an 80-year-old man and a 45-year-old woman. In both cases, the clinical presentation was that of a serosal tumor, including effusions and pleural thickening. In the former, the serosal infiltration raised the suspicion of mesothelioma reinforced by history of occupational exposure to asbestos. Patient general condition deteriorated rapidly. Postmortem examination revealed unilateral encasing of the lung within a thick, irregular neoplastic rind. In addition, tumoral involvement was seen in the homolateral third rib and the clavicle. Histologic examination of pleural masses demonstrated diffuse infiltration by highly atypical, pleomorphic plasma cells with κ chain restriction. In the second case, clinical presentation was also suspicious of mesothelioma. Nonetheless, a pleural biopsy specimen showed irregular sheets of plasma cells showing κ light chain restriction. A bone marrow aspirate was also positive for abnormal plasma cell infiltrates. Despite chemotherapy, the patient died 4 months after presentation. Although rarely, it seems that plasmacytoma may present with an exclusive or preponderant pleural involvement; and it may therefore be added to the list of pseudomesotheliomatous tumors.