Extramedullary Plasmacytoma frequently remain localized and display lower incidence of conversion to multiple myeloma than do solitary plamacytoma of bone. If histologic diagnosis of plasmacytoma is comfirmed, all screening tests to rule out multiple myeloma is neccesary before initial treatment and during follow-up period. Histologic diagosis of plasmacytoma of maxillary sinus was made in two cases. Radiotherapy was given post-operatively. In first case, serum and urine electrophoresis demonstrated monoclonal gammopathy, other tests including bone marrow biopsy and X-ray survey in order to rule out multiple myeloma were not performed in first patient. In second case, all screenig tests were negative. The two patients complained of bone pain. So, serum and urine electrophoresis, X-ray survey, bone marrow aspiration biopsy were performed, the findings were consistent with multiple myeloma. Two cases were diagnosed as extramedullary plamacytoma converted to multiple myeloma. Chemotherapy was begun, continued to 15th cycle in first patients 19th cycle in second patients.