We report a 60-year-old man with prostate cancer diagnosed during androgen replacement therapy (ART) for late onset hypogonadism after surgery for pituitary adenoma. He was refered to the department of urology since prostate specific antigen values were elevated after 6 months of ART. After the diagnosis of prostate cancer, ART was discontinued, and robot-asssited laparoscopic radical prostatectomy with pelvic lymphadenoctomy was performed. Pathological examination revealed Gleason score 4 + 5 prostate adenocarcinoma with seminal vesicle invasion and lymph node metastasis(pT3bN1). He has stayed biochemically and radiologically disease-free 33 months postoperatively.