Abnormalities of p53, a tumor suppressor gene, have been considered to play an important role in tumorigenesis. Clinically, overexpression of p53 has been reported to correlate with poor prognosis in several types of tumors. In this study, we examined 221 cases of endometrioid endometrial carcinoma for overexpression of p53 using immunohistochemistry in patients with a median follow-up of 41 months. Immunohistochemical analysis was performed with monoclonal antibody pAb1801. Overexpression of p53 was detected in 47 of 221 cases (21.3%). There was a statistically significant correlation between p53 overexpression and poor prognosis (P < .0001). In the early stages of cancer (stages 1 and 2), p53 was overexpressed in 11 of 22 patients (50%) who died or had a recurrence during follow-up. In contrast, overexpression was detected in only 14.7% of the 156 disease-free patients during the same period (P < .0001). In advanced stages (stages 3 and 4), tumors from patients with recurrent disease had a higher frequency of overexpression (41.2%; 7 of 17) than those of disease-free patients (23.1% 6 of 26). However, the difference between these frequencies was not statistically significant. In multivariate analysis using the Cox proportional hazard model, p53 overexpression was an independent risk factor when compared with clinical stage, nuclear grade, and patient age. Our results indicate that p53 immunohistochemical evaluation of the most common form of endometrial cancer may be useful in identifying cases of aggressive carcinoma, especially in the early stages.