A 72-year-old woman presented with a progressive swollen and painful left knee over a period of a month. The patient had a history of tuberculous arthritis of the left knee joint at the age of 1-2 years. Since then the knee showed a deformity and contraction. MRI scan demonstrated a soft tissue tumor measuring 7 x 8 x 6 cm in the front of the knee, a biopsy specimen taken from which revealed large B-cell lymphoma. The tumor cells showed a phenotype of CD3- CD5- CD10- CD20+ CD21- BCL2+, and surface immunoglobulin M-kappa. Since the lymphoma cells were found to be negative for Epstein-Barr virus encoded RNA by means of in situ hybridization, the DLBCL in this patient was thought to be caused by a different mechanism from pyothorax-associated lymphoma. The patient was treated with chemotherapy (CHOP) followed by radiotherapy, and obtained partial remission. There is no evidence of progression seven months after diagnosis.