A 62-year-old man visited our hospital in July 1993, because of a right thigh mass which had grown gradually since two years previously. Physical examination revealed that the mass at the right thigh region, was elastic soft and about 15 x 10 cm in diameter, without regional lymph node swelling. An ultrasound study showed a hypoechogenic and mesh patterned mass. MRI revealed that the tumor was well defined from subcutaneous adipose tissue and skeletal muscle, indicating that it arose in connective tissue. Angiography demonstrated diffuse hypervascularization of the tumor, and Gallium scintigraphy showed remarkable accumulation at the tumor. Serum IgM was increased, which was proven to be an monoclonal hypergammopathy (IgM, lambda). Histological examination of a biopsied specimen obtained from the thigh mass revealed B cell lymphoma, lymphoplasmacytic cell type. The patient achieved a complete remission after surgical treatment following radiation and combination chemotherapy.