We investigated the frequency, histopathologic, and immunohistochemical characteristics of tubal involvement in uterine serous carcinoma (USC) and aimed to clarify the relationship between "serous tubal intraepithelial carcinoma (STIC)" and USC in these cases. Cases of USC with complete tubal examination were prospectively collected and reviewed for the presence of tubal involvement. Immunohistochemical analysis for p53 and WT1 was performed on the endometrial and tubal tumor in cases with tubal involvement. Of 161 USC cases (pure USC or a component of a mixed carcinoma or a carcinosarcoma), 32 (20%) showed tubal involvement (unilateral: n=19; bilateral: n=13). The uterine tumors in cases with tubal involvement showed a trend toward increased likelihood of deep myometrial and lymphovascular invasion (LVI) compared with those without tubal involvement. The tubal fimbriae were involved in 15/32 cases. Tubal involvement was mucosal in 30/32 cases, mural in 14/32, serosal in 5/32, invasive in 22/32, and there was LVI in the tube in 13/32. STIC-like features were seen in 17/32 cases (7 as the only pattern of involvement, 9 with associated invasive carcinoma, and 5 with LVI). Immunostaining showed complete concordance of p53 and WT1 between the endometrial and tubal tumors in 26/32 cases, the majority being WT1 negative or only focally positive (19/26), and all exhibiting mutation-type p53 staining. On the basis of the histologic and immunohistochemical features, the tubal tumor was considered to represent metastatic USC in 26/32 cases, most likely metastatic USC in 2/32 cases, an independent tubal primary tumor in 3/32 cases, and to be of uncertain origin in the 1 remaining case. STIC-like lesions were considered to represent metastatic USC in 12/17 cases, most likely metastatic USC in 2/17 cases, an independent tubal primary in 2/17 cases, and of uncertain origin in the 1 remaining case. Tubal involvement, including STIC-like lesions, is seen in one fifth of USC when the tubes are examined in their entirety. The tubal involvement is metastatic in the vast majority of cases. Immunohistochemical studies assist, in most cases, in confirming the metastatic nature of the tubal disease. Consideration should be given to completely examining the fallopian tubes in apparent stage I or II USCs, as this will result in upstaging in a significant minority of cases.