A dermal leiomyosarcoma associated with a still asymptomatic osteomedullary plasmocytoma with IgA paraproteinemia develop-d rapidly in a 78-year-old man. The same paraprotein type IgA kappa was identified, with distinct decrease in concentration, in serum, sarcoma-tissue, and in the tumour-surrounding skin area by m-ans of immunoelectrophoresis, radial immunodiffusion, and Ouchterlony test. The immunochemical identity of the paraprotein, the course of the disease as followed clinically and immunochemically, and several histological criteria are in favour of monoclonal origin of the paraprotein from malignant plasmocytoma cells. The accumulation of paraprotein in the sarcoma tissue is primarily explained by the marked blood congestion of the tumour. To our knowledge no report exists in the medical literature of an association of dermal leiomyosarcoma and early paraproteinemic plasmocytoma. This coincidence may be only of chance but we rather suggest an immunopathological relation of both tumors due to a partial immune insufficiency caused by the preceding plasmocytoma, resulting in a diminished immunological "surveillance" of the organism and favouring the development of a new malignant cell population (i.e. leiomyosarcoma).