A case of plasmacytoma occurring in the maxilla with transition into multiple myeloma is reported. The patient was a 69-year-old man who had swelling of the right side of the maxillary alveolar process. A biopsy was performed, and the histological diagnosis was plasmacytoma, predominantly IgA-κtype. Because there were no abnormal laboratory findings in serum, urine, bonem arrow, or bones at other regions, the patient underwent radiotherapy to the maxilla. Consequently, the tumor markedly regressed, Since an antrostomy revealed remaining tumor cells, additional radiotherapy was performed. Immediately after radiotherapy, bone scintigraphy demonstrated abnormal uptake throughout the body, including the thoracic and lumbar bones. These findings together with MRI and laboratory test results indicated a transition into multiple myeloma. The patient is now receiving chemotherapy with cyclophosphamide and prednisolone. There is no sign of local tumor recurrence.
