Prostate cancer is a common cancer in men, and its clinical behavior ranges from microscopic tumors to aggressive cancer with metastatic potential. We report the case of an 81-year-old man with primary prostate cancer, AJCC stage T2 and Gleason's score 10, which was diagnosed in March 2007. The patient received upper endoscopy due to symptoms of abdominal fullness and body weight loss, and a soft tumor in the secondary portion of the duodenum was found. Biopsy demonstrated a poorly differentiated carcinoma with positive prostate specific antigen cytoplasmic stain. The pathologic appearance was consistent with prostate cancer metastatic to the small bowel. Abdominal computed tomography also showed multiple liver tumors with enlarged retroperitoneal lymph nodes. This case should help raise clinical awareness of the possibility of unusual metastatic sites in patients with primary prostate cancer.