Most nonuterine high-grade serous carcinomas (HGSCs) in women with hereditary breast and ovarian cancer syndrome, due to germline BRCA1/2 mutation, arise in the fimbria of the fallopian tube. However, the site of origin of sporadic HGSC, which is usually widely disseminated at presentation, is not well established. We sought to characterize cases of HGSC discovered incidentally in patients not known to be at high risk, in order to determine the site distribution and possible origin of sporadic HGSC. Incidental microscopic, non-mass-forming cases of serous tubal intraepithelial carcinoma or HGSC in salpingo-oophorectomy specimens in which the tubes and ovaries had been extensively examined were identified. No patients were known or suspected BRCA1/2 mutation carriers. Twenty-one cases were identified (mean age: 57 y). Surgery was for benign disease (n=15), uterine endometrioid adenocarcinoma or atypical hyperplasia (n=3), bladder carcinoma (n=1), or ovarian serous borderline tumor (n=2). In 16 of 21 cases, the lesion was confined to the fallopian tube (unilateral in 14 cases, bilateral in 2). There was serous tubal intraepithelial carcinoma in all cases and invasive HGSC into the underlying lamina propria in 8 of these 16 cases; the invasive focus measured 1.3 cm or less in every case. In the remaining 5 cases, there was fallopian tube mucosal and ovarian involvement; in 2 of these cases, there was also microscopic peritoneal involvement. Sporadic cases of nonuterine HGSC arise in the fallopian tube fimbria in a large majority of cases, providing further evidence for the tubal origin of these neoplasms.