Seventy-nine ovarian serous and mucinous borderline tumors, 36 stage I carcinomas and 39 stage II-IV carcinomas were studied for p53 protein accumulation with monoclonal antibody PAb1801.p53 protein was expressed in 14% of borderline tumors, 36% of stage I carcinomas, and 64% of higher stage carcinomas. All immunopositive carcinomas accumulated p53 protein in the primary tumor, and 95% of them showed concordance in staining among different tissue blocks. A difference in frequency of p53 protein accumulation between stage I and higher stage serous carcinomas was not statistically significant. p53 positivity was associated with microinvasion, microcarcinoma and coexistent carcinoma in mucinous borderline tumors (P = .025). An association between p53 protein expression and poor tumor differentiation in Stage I carcinomas as statistically significant (P = .03). p53 positivity was observed in a poorly differentiated endometrioid carcinoma as well as in adjacent benign endometriotic tissue. These results suggest that p53 abnormalities may be early events in ovarian cancer, possibly contributing to malignant transformation of some borderline tumors, endometriosis and other carcinoma precursors.