Fifty carcinomas that were partially to completely papillary in nature were examined. According to urethroscopic and rectal palpation findings, six of the carcinomas were located centrally, 40 tumors were in the prostate proper, and four were clinical stage T0. The epithelium of the papillary portions of the tumors was dark in some instances, light in others. Immunohistochemistry revealed that 20 of 22 tumors were positive for prostatic acid phosphatase (PAP) and prostate-specific antigen (PSA). In no case was a topical relationship to the utriculus prostaticus demonstrable. The epithelium of the utriculus in seven additional patients who were not involved in this series also stained positively for PAP and PSA. Usual carcinomas of the prostate proper can develop endometrioid structures that do not differ immunohistochemically from ordinary portions of the carcinoma. Tumors located in central portions of the prostate are, in our opinion, morphologic variants of usual prostatic carcinomas, and apparently arise in prostatic ducts. We conclude that a distinction between endometrioid carcinomas and tumors of prostatic ducts does not seem justified and that papillary prostatic carcinomas should be treated like common prostatic cancer.