We present a case of malignant lymphoma of the prostate. A 73-year-old man was admitted due to constipation and urinary retention for several days. After a serious evaluation of the genitourinary and gastrointestinal tracts, CT scan showed a huge homogeneous, soft tissue mass in the right lower pelvic cavity with lymph nodes enlargement at left iliac and right common femoral regions, while indistinct margin related to the anterior wall of the rectum was noted. Transurethral resection of the prostate was performed and pathology showed diffuse large B-cell lymphoma. Literatures are also reviewed. (J Urol R.O.C., 9:21-24,1998)