A 64-year-old male was referred to our hospital with a prostate tumor accompanying a huge prostatic cyst. He had been followed up for 10 years by another urologist, and received several prostate biopsies, which failed to make a definite diagnosis. We carefully reviewed the biopsy slides and diagnosed the lesion as prostatic stromal tumor of uncertain malignant potential (STUMP). We performed radical prostatectomy. From the excised specimen, we confirmed the diagnosis of prostatic STUMP. Eight months later, there was no sign of metastasis or recurrence.