The morphologic, immunophenotypic, and cytogenetic features of solitary plasmacytomas of bone are identical to those of multiple myeloma. The image shows a somewhat homogenous population of mature plasma cells from a case of solitary plasmacytoma involving a rib. Occasional bi- and tri-nucleated cells are present. The better-differentiated lesions like the one shown here are easily recognized as plasmacyctomas. The less well-differentiated cases have to be distinguished from metastatic tumors, lymphomas with plasmacytic differentiation, Langerhans cell histiocytosis, and some primary bone lesions.