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It has been proposed that the presence of tubal intraepithelial carcinoma (TIC), in association with one-third to nearly half of pelvic serous carcinomas, is evidence of fallopian tube origin for high-grade serous carcinomas that would have been otherwise classified as primary ovarian or peritoneal. To address this hypothesis, we evaluated a series of 114 consecutive pelvic (nonuterine) gynecologic carcinomas at our institution (2006 to 2008) to determine the frequency of TIC in 52 cases in which all the resected fallopian tube tissue was examined microscopically. These 52 cases were classified as ovarian (n=37), peritoneal (n=8), or fallopian tube (n=7) in origin as per conventional criteria based on disease distribution. The presence of TIC and its location and relationship to invasive carcinoma in the fallopian tubes and ovaries were assessed. Among the 45 cases of ovarian/peritoneal origin, carcinoma subtypes included 41 high-grade serous, 1 endometrioid, 1 mucinous, 1 high-grade, not otherwise specified, and 1 malignant mesodermal mixed tumor. TIC was identified in 24 cases (59%) of high-grade serous carcinoma but not among any of the other subtypes; therefore, the term serous TIC (STIC) is a more specific appellation. STICs were located in the fimbriated end of the tube in 22 cases (92%) and in the ampulla in 2 (8%); they were unilateral in 21 (88%) and bilateral in 3 (13%). STICs in the absence of an associated invasive carcinoma in the same tube were detected in 7 cases (30%) and with invasive carcinoma in the same tube in 17 (71%). Unilateral STICs were associated with bilateral ovarian involvement in 15 cases and unilateral (ipsilateral) ovarian involvement in 5 (the remaining case with a unilateral STIC had a primary peritoneal tumor with no ovarian involvement); the bilateral STICs were all associated with bilateral ovarian involvement. Six of the 7 primary tubal tumors were high-grade serous carcinomas, and 4 of these 6 (67%) had STICs. Based on conventional criteria, 70%, 17%, and 13% of high-grade serous carcinomas qualified for classification as ovarian, peritoneal, and tubal in origin, respectively; however, using STIC as a supplemental criterion to define a case as tubal in origin, the distribution was modified to 28%, 8%, and 64%, respectively. Features of tumors in the ovary that generally suggest metastatic disease (bilaterality, small size, nodular growth pattern, and surface plaques) were identified with similar frequency in cases with and without STIC and were, therefore, not predictive of tubal origin. The findings, showing that nearly 60% of high-grade pelvic (nonuterine) serous carcinomas are associated with STICs, are consistent with the proposal that the fallopian tube is the source of a majority of these tumors. If these findings can be validated by molecular studies that definitively establish that STIC is the earliest form of carcinoma rather than intraepithelial spread from adjacent invasive serous carcinoma of ovarian or peritoneal origin, they will have important clinical implications for screening, treatment, and prevention.