Primary plasmacytoma of the skin without evidence of bone marrow plasmacytosis is a rare disorder belonging to the heterogenous spectrum of plasma cell neoplasms. With immunohistochemical techniques, differentiation from benign plasma cell aggregates can be accomplished by demonstrating the monoclonality of tumor cells. We describe a patient in whom a solitary primary cutaneous plasmacytoma developed on the left thigh. Immunohistochemically, plasma cells showed restriction of immunoglobulin lambda-chain expression. Underlying multiple myeloma was excluded by serum protein and immunoglobulin electrophoresis, roentgenographic skeletal survey, and bone marrow biopsy. The tumor responded well to local electron-beam radiotherapy. Whereas multiple primary cutaneous plasmacytomas seem to carry a significant early mortality rate, the prognosis of solitary lesions is likely to be more favorable.