An 89-year-old man presented with urinary retention. Digital rectal examination was benign despite elevated serum prostate-specific antigen (PSA) level. Suprapubic prostatectomy was performed under the diagnosis of benign prostatic hyperplasia. Histopathological diagnosis was endometrioid carcinoma showing positive immunohistochemical staining for PSA. Postoperatively, estrogen was administered for 9 months. After 37 months, serum PSA level began to increase and a palpable nodule was detected on digital rectal examination. Biopsy showed coexistence of endometrioid carcinoma and acinar adenocarcinoma. Hormonal treatment was resumed and the disease has been well controlled for 65 months postoperatively.