Despite earlier arguments that the so-called "endometrioid carcinoma" is of Müllerian remnant origin, our clinicopathologic study of 35 cases indicated that it is actually an adenocarcinoma of prostatic duct origin. With respect to histology, it has two growth patterns; type A, an exuberant papillary endometrioid pattern with a focal intraductal component, and type B, less papillary-endometrioid growth and more intraductal components. Immunoperoxidase study showed immunoreactivity for prostatic-specific antigen and prostatic acid phosphatase in all 20 cases tested. Ultrastructural study identified prostatic epithelial cells rather than ciliated endometrial features. In 18 of 35 cases, we identified microacinar carcinoma of the prostate, MDAH grade I, Gleason's combined score 2, 3, or 4. Twenty-two treated patients presented with obstruction, hematuria, or both; 20 had stage C or D disease, suggesting that this tumor is more aggressive than was originally assumed.