The authors reviewed the histologic slides of 2600 prostatic carcinomas seen at Memorial Hospital from 1963 to 1983. In ten cases, resection specimens had a predominantly endometrioid appearance. Six patients had polypoid lesions in and around the verumontanum, and one had a polypoid lesion away from the verumontanum. Two patients had no mucosal lesions and one was not cystoscoped. Histologically, the tumors showed a tall pseudostratified columnar epithelium, usually with amphophilic cytoplasm. The cells were arranged either along papillae or in complexes of large acini or in single glands. In eight of the ten cases, the endometrioid carcinomas were associated with a prior or coexistent typical microacinar prostatic adenocarcinoma. In four cases, the endometrioid pattern existed in a pure form, although in two such cases with urethral tumors, the patients had histories of successfully treated microacinar adenocarcinomas of the posterior prostatic lobe. In one case, a urethral endometrioid tumor coexisted with a small posterior lobe microacinar adenocarcinoma. In five cases, both endometrioid and microacinar carcinomas were seen, including endometrioid and microacinar carcinomas found at the same site at different times (2 cases), tumors with a predominantly endometrioid, yet focally microacinar pattern (1 case), and primary tumors where lymph node metastases had different histologic features (2 cases). Of the three patients with a pure or predominantly endometrioid pattern treated with diethylstilbestrol, two had a marked clinical response. All ten endometrioid prostatic adenocarcinomas showed prostate-specific antigen and prostate-specific acid phosphatase immunoreactivity, in contrast to none of the control uterine endometrial carcinomas. In material spanning a 20-year period, the authors have not seen a single prostatic tumor entirely analogous to the uterine endometrial carcinoma. Until such proof exists, prostatic carcinomas with endometrioid features are best classified and treated as variants of prostatic duct carcinomas.