We report a case of a 64-year-old patient in whom limitation of mouth opening was the presenting symptom of plasmacytoma. Intra-oral biopsy confirmed a plasma-cell tumour, with no sign of extension on imaging. He was treated with radiotherapy (4000 cGy) and followed up. The diagnosis of solitary plasmacytoma of bone can be confirmed only when the presence of systemic disease has been excluded by clinical, biological, and radiological investigations. We think that the treatment should consist only of radiotherapy. Long-term follow-up is necessary because of the high risk of development of multiple myeloma, which may be delayed.