A 32-year-old man was admitted to our hospital because of lumbago and an abdominal mass revealed by abdominal ultrasonography. Abdominal CT scan and MRI revealed multiple para-aortic lymph node swelling involving several arteries and veins. As there was no superficial lymph node swelling, percutaneous lymph node biopsy was performed under ultrasonographic guide. Although non-Hodgkin's lymphoma, diffuse, small cell type was suspected by light microscopic study, the monoclonality of the lymphocytes in the obtained specimen was not clear by the immunohistochemical study. Southern blot hybridization analysis of the biopsy specimen revealed the rearrangement of IgH and IgL (lambda) chain gene, indicating the existence of monoclonal proliferation of lymphoma cells. The DNA analysis appears useful for the differential diagnosis of lympho-proliferative diseases.