We have experienced a case of retroperitoneal plasmacytoma. A 47-year-old man was admitted to our hospital with the chief complaints of right lumbago and lower abdominal dull pain. Roentgenography revealed retroperitoneal tumors, and right nephrectomy and excision of left pararenal tumors were performed. The histological examination and urine immunoelectrophoresis showed retroperitoneal plasmacytoma. No signs or symptoms of multiple myeloma were recognized postoperatively. The literature is reviewed briefly concerning the diagnosis and treatment of retroperitoneal plasmacytoma.