Endometrial serous carcinoma (ESC) and FIGO (International Federation of Gynecology and Obstetrics) grade 3 endometrioid adenocarcinoma (EC) are high-grade endometrial tumors that have different clinical and morphologic attributes. Alteration of p53 tumor suppressor protein function has been implicated in the pathogenesis of both tumors, although the mechanisms may differ. We sought to investigate this difference by comparing immunohistochemical expression of p53 and mdm2. p53 immunoreactivity often correlates with gene mutation, whereas increased mdm2 expression is linked to complex formation with wild-type p53 resulting in its inactivation. Twenty cases of ESC and 21 cases of EC were evaluated and an immunoreactivity score (IRS) was assigned using both the percentage of cells stained and the intensity of staining. The overall IRSs were significantly different in ESCs versus ECs for both p53 and mdm2 (p <0.001 and p <0.01, respectively). Strong mean immunoreactivity for p53 was detected in 15 (75%) ESCs as compared to only weak mean immunoreactivity in 17 (81%) ECs. Conversely, for mdm2 expression, 17 (81%) ECs had moderate mean immunoreactivity whereas 9 (45%) ESCs showed only weak mean immunoreactivity. mdm2 expression more closely correlated with p53 expression in ECs than in ESCs. In ECs, mdm2 was detected in 16 of 17 (94%) p53-positive tumors but in only 1 of 3 (33%) p53-negative tumors (p <0.025). In ESCs, mdm2 was detected in 9 of 15 (60%) p53-positive tumors but in none of the 5 p53-negative tumors (p <0.10). Overall, our results demonstrate an inverse relationship between the expression of p53 and mdm2 in ESC versus high-grade EC. Specifically, strong p53 immunoreactivity is associated with weak mdm2 expression in ESC and weak p53 expression is associated with moderate mdm2 expression in EC. These results suggest different pathogenetic pathways resulting in loss of normal p53 function in these two tumors: by p53 gene mutation (strong p53 overexpression) in ESCs, or by mdm2 complex formation and inactivation of p53 in high-grade ECs.