Mutations of the p53 gene on chromosome 17p are a common genetic change in the malignant progression of many cancers. We have analyzed 38 malignant tumors of ovarian or peritoneal müllerian type for evidence of p53 variations at either the DNA or protein levels. Genetic studies were based on single-strand conformation polymorphism analysis and DNA sequencing of exons 2 through 11 of the p53 gene; mutations were detected in 79% of the tumors. These data show a statistically significant association between mutations at C.G pairs and a history of estrogen therapy. Two of 20 patients whose normal tissue could be studied carried germ-line mutations of p53. Immunohistochemical analysis of the p53 protein was carried out using monoclonal antibody PAb1801. Ninety-six percent of the missense mutations were associated with abnormal accumulation of p53 protein, but nonsense mutations, a splicing mutation, and most deletions did not result in p53 protein accumulation. A statistically significant association between p53 protein accumulation in poorly differentiated stage III serous carcinomas and small primary tumor size at diagnosis was found, perhaps suggesting that p53 protein accumulation accelerates the metastatic spread from a primary tumor. Overall, our findings indicate that alterations of p53 play a major role in ovarian cancer, including predisposition to the disease in some patients, and suggest a possible mechanism for somatic mutations leading to this cancer.