Hobnail-like cells, which suggest a diagnosis of clear cell carcinoma, are also focally observed in serous borderline tumor of the ovary, causing diagnostic confusion. However, the precise nature of hobnail-like cells in serous borderline tumor has not been well characterized. The purpose of this study was to clarify whether or not hobnail-like cells in serous borderline tumor represent concomitant incipient clear cell neoplasms. First, we carefully reviewed hematoxylin and eosin slides taken from 115 ovarian tumors diagnosed as clear cell carcinoma (73 cases), mixed adenocarcinoma containing clear cell carcinoma (5 cases), and serous borderline tumor (37 cases) to clarify the frequency of coexistence of typical clear cell carcinoma and serous borderline tumor. Through the hematoxylin and eosin review, we paid special attention to the cytologic features of hobnail-like cells in serous borderline tumor and serous borderline tumor-like papillary areas in clear cell carcinoma. Second, we selected 19 serous borderline tumors and 16 clear cell carcinomas, in which hobnail-like cells were easily recognizable, and investigated the immunohistochemical expression of estrogen receptor and Wilms tumor gene protein. No coexistence of clear cell carcinoma and serous borderline tumor was evident in any of the above 115 ovarian tumors. Hobnail-like cells were focally positive for estrogen receptor and Wilms tumor gene protein in nearly all serous borderline tumors. Hobnail-like cells in all clear cell carcinomas were completely negative for estrogen receptor and Wilms tumor gene protein, although estrogen receptor expression was very focally observed (less than 5% area) in non-hobnail cells of only one clear cell carcinoma. In conclusion, hobnail-like cells in serous borderline tumor do not represent concomitant incipient clear cell neoplasms because (1) clear cell carcinoma and serous borderline tumor do not coexist and (2) hobnail-like cells in clear cell carcinoma and serous borderline tumor are immunophenotypically distinct. Recognition of our conclusion may protect a patient with "conspicuous hobnail-like cells in serous borderline tumor" from an erroneous overdiagnosis of "concomitant clear cell carcinoma admixed with serous borderline tumor."