Solitary plasmocytoma is a rare plasma cell disorder as compared with multiple myeloma. In order to evaluate their relationships, the clinical findings in 51 patients with solitary plasmocytoma were analyzed. The median follow-up was 7.8 years (range 3 to 14). Twenty patients (39%) developed multiple myeloma in a average of 3.8 years. The risk factors associated with the development of multiple myeloma were an initial abnormal electrophoresis (M peak), age (less than 40 years) and an increase in beta-2-microglobulin. Although radiotherapy remains the initial treatment of solitary plasmocytoma, a careful evaluation should be performed in order to consider chemotherapy in selected patients with high risk of developing multiple myeloma.