Primary cutaneous plasmacytoma is a rare type of cutaneous B-cell lymphoma that arises primarily in the skin, and this is derived from clonally expanded plasma cells with various degrees of maturation and atypia. A 72-year-old man had an asymptomatic, solitary reddish to violaceous nodule on the back for about 4 months. The histologic finding of the skin biopsy specimen demonstrated an infiltration of variably matured plasma cells in the dermis, and these cells showed a monotypic expression of immunoglobulin kappa chains on immunohistochemical staining. Staging investigations excluded any extracutaneous manifestations of the disease. Only a few cases of primary cutaneous plasmacytoma have been published in the Korean dermatologic literature. Herein, we report on an interesting case of primary cutaneous plasmacytoma with monoclonality of kappa chains. (Korean J Dermatol 2010;48(12):1114∼1117)