Uterine serous carcinoma (USC) is an uncommon but aggressive type of endometrial cancer associated with rapid progression of disease and a poor prognosis. Both USC and its recently described putative precursor, endometrial intraepithelial carcinoma (EIC), demonstrate strong p53 overexpression by immunohistochemistry, suggesting alteration of the p53 gene in their pathogenesis. In the present study, we evaluated 21 USCs and 9 EICs for mutations in the p53 gene using direct sequence analysis and found that 90% of USCs and 78% of EICs contain mutations. Significantly, mutations were found in 3 cases of EIC without associated invasive carcinoma and identical mutations were detected in cases with synchronous USC and EIC. Strong p53 immunoreactivity was seen in the majority of USCs and EICs and correlated with p53 gene mutation, although lack of reactivity did not always indicate the absence of a gene mutation. Loss of heterozygosity of chromosome 17p was observed in 100% of USCs and in 43% of EICs, demonstrating that loss of the wild-type p53 allele occurs early in the development of serous carcinoma. Overall, our results reveal that p53 mutations are very common in USC and EIC. The presence of p53 gene mutations in EIC further suggests that p53 alteration plays an important role early in the pathogenesis of serous carcinoma, possibly accounting for its aggressive biological behavior.