To probe the mechanism of the development of ovarian cancer from endometriosis, which is a multistep process that involves the activation of oncogenes and inactivation of tumor suppressor genes, we evaluated p53 mutations in solitary endometriosis and endometriosis coexisting with ovarian clear cell carcinoma (OCCA) and ovarian endometrioid carcinoma (OEC). We examined 7 cases of solitary ovarian endometriosis, 13 cases of OCCA, and 9 cases of OEC. Cancer tissue specimens that also contained endometriosis without atypia were chosen. Using a laser microdissection system, epithelial cells from the areas of endometriosis were isolated, and DNA was extracted. We amplified exons 5, 6, 7, and 8 of the p53 gene, and direct sequencing was performed. p53 mutation was observed in 4 (30.8%) of 13 specimens of endometriosis coexisting with OCCA, whereas no mutations were detected in solitary endometriosis or endometriosis coexisting with OEC. We thought that some genetic alterations, which induce p53 mutations in endometriosis, may affect malignant transformation of endometriosis into OCCA.