Based on urological statistic results, finding a cyst in prostate are not common cases. The size of cysts are usually smaller than prostate and they are also generally found within prostate or seminal vesicles. Nevertheless, we have experienced a case of a large prostaic cystadenoma which is misdiagnosed as a mullerian duct cyst. A 55-year-old male visited Ewha Woman's University Hospital with the symptom of residual urine sensation and no other specific symptoms. We could find a large multiple septated cystic mass in his pelvis by CT scan. The large multiple septated cystic mass seemed a Mullerian duct cyst. But after surgical excision, the department of pathology noticed us it was a prostatic cystadenoma. Presently, we report this case with a review of the mullerian duct cyst and other cyst associated with prostate.