A case of extramedullary plasmacytoma of the lung associated with nodular immunoglobulin deposits is presented. The main tumor was located in the posterior basal segment of the lower lobe of the left lung and showed intrabronchial polypoid growth. Multiple metastatic tumors were observed in regional lymph nodes and the visceral and parietal pleura. Histologically, the tumor was composed of sheets of plasma cells with mild atypia. A large amount of amorphous material resembling amyloid was observed in the tumors, and was more marked in the metastatic lesions. Immunohistochemically, the tumor cells were shown to contain monoclonal IgG-kappa immunoglobulin. The amorphous deposits were not identical to amyloid since congo red stain was negative and amyloid fibrils were not observed ultrastructurally. By Western blotting, immunoglobulin (IgG, kappa) was identified as the major component in an extract from the nodular deposits. In the preoperative serum from the patient, M-protein (IgG-kappa) was detected by immunoelectrophoresis. Bone marrow examination revealed no abnormality.