Multilocular prostatic cystadenoma is a rare benign neoplasm located between the bladder and the rectum. These are prostatic tissue and have been shown to harbor high-grade intraepithelial neoplasia and likely susceptible to the same disease processes seen in the prostate gland. We report the first case of conventional prostatic adenocarcinoma involving a multilocular cystadenoma. Distinction from cystadenocarcinoma is also made. © 2010 Japanese Society of Pathology and Blackwell Publishing Asia Pty Ltd.