Criteria for the very rare diagnosis of primary seminal vesicle carcinoma have traditionally been highly stringent but may be relaxed with the application of immunohistochemistry to the diagnosis of mass lesions that occur in the male pelvis. The authors report a case of disseminated carcinoma with a clinically occult primary site that apparently had its origin in the seminal vesicle. Autopsy revealed a 10-cm tumor enveloping the prostate and seminal vesicles without involvement of colonic or urothelial mucosa. Much smaller tumors were present in other sites outside the pelvis. The tumor was composed of poorly formed glands and sheets of malignant-appearing cells, involved the seminal vesicle, and had the immunohistochemical profile of seminal vesicle carcinoma, notably strong immunoreactivity for CA-125 and no immunoreactivity for cytokeratin-20 or prostate-specific markers.