Pelvic serous cancer is a diverse disease, and the assignment of primary site -- ovarian, tubal, or peritoneal -- is often problematic. Recent studies indicate that a proportion of these tumors arise from the distal fallopian tube, originating as serous tubal intraepithelial carcinoma (STIC). This study examined the relationship of 2 parameters for assigning origin -- endosalpingeal involvement and dominant ovarian mass -- in the context of STIC. Endometrioid carcinomas served as a reference. Eighty-seven consecutive pelvic serous cancers in which the tubes and ovaries were completely examined (SEE-FIM protocol) were analyzed. The presence of a dominant ovarian mass (DOM+), involvement of the fimbrial mucosa (FIM+), and STIC were correlated. In addition, tumor categories were compared with respect to PAX8, p73, p53, and p16 immunohistochemistry. Of the 27 DOM+ cases, 13 (48%) were FIM+ and a STIC was present in 3 (11%). Of the 60 DOM(-) cases, 48 (78%) were FIM+ and 28 (45%) harbored a STIC. In 92% of all cases, tumor distribution was extensive with bilateral ovarian and extraovarian peritoneal involvement. All tumor categories were immunophenotypically similar. In contrast, DOM+, FIM+, and STIC were found in 81%, 19%, and 0% of ovarian endometrioid carcinomas. In conclusion, there is a significant inverse relationship between DOM+ and STIC (P=0.001), indicating both parameters are of value in grouping pelvic serous carcinomas more likely to be ovarian [DOM+/FIM(-)] versus fimbrial [DOM(-)/STIC], and ovarian or peritoneal surface (DOM-/FIM-) in origin. Nevertheless, the shared immunophenotype suggests a common cell of origin for all categories, irrespective of site.