The clinical and histopathological features of a case of solitary extramedullary plasmacytoma of the breast are described. After 46 mth of follow-up, there has been no recurrence of the tumour, and no evidence of further extramedullary plasmacytomas, multiple myeloma or diffuse myelomatosis either clinically, radiologically or on biochemical and haematological investigations. She has developed diabetes mellitus. Distinction is made between the true extramedullary plasmacytoma and those which are a manifestation of multiple myeloma. The unpredictable behaviour and prognosis of extramedullary plasmacytomas is indicated.