The diagnostic distinction between endometrial serous carcinoma (ESC) and endometrial clear-cell carcinoma (CCC) may occasionally be problematic, and one potentially contributing factor is the finding of clear cells in otherwise classic cases of ESC. This study aimed to define the frequency of this finding and comparatively assessed the immunophenotype of the clear cells. A review of 56 cases of ESC identified 8 (14.28%) with clear cells, representing 1% to 20% (median 7.5) of tumoral volume in these cases. In only 3 cases were clear cells discernible at low (×20) magnification. There was no significant difference in stage distribution or age between ESC patients with and without clear cells. The immunophenotypes of ESC-associated clear cells (group 1) were compared with foci of conventional ESC on another tissue block within the same case (group 2; n = 8) as well as a randomly selected cohort of CCC cases (group 3; n = 8). Groups 1 and 2 showed no significant differences regarding p53, ER, PR, Napsin-A, p504S, and hepatocyte nuclear factor 1β (HNF1β) expression, or regarding mitotic indices or Ki67 proliferation rate. In contrast, group 1 cases showed an immunophenotypic profile that was notably different from that of group 3 cases, with the former showing statistically significantly higher/more frequent expression of ER, PR, Ki67, and p53 and lower/less frequent expression of Napsin-A, p504S, and HNF1β. We conclude that clear-cell change is seen in 14% of ESCs and is discernible at low magnification in only 5%; these areas show an immunophenotype that is essentially identical to the associated background conventional ESC and are phenotypically dissimilar to CCC.