The origin of malignant ovarian epithelial tumors is uncertain and has been the subject of considerably controversy. Some favor the theory of origin in precursor lesions such as benign cystadenomas or tumors of low malignant potential (LMP; borderline tumors), whereas others favor the concept of an independent origin of carcinomas from the ovarian surface epithelium or inclusion cysts. Recently, the demonstration of identical molecular alterations in morphologically benign and malignant areas within the same ovarian tumor have suggested the possibility that the malignant epithelium had undergone differentiation to a benign appearance. Because both areas were present in the same tumor, however, the possibility of progression of the morphologically benign component could not be excluded. We present a case of simultaneous mucinous carcinoma and contralateral tumor of LMP which exhibited identical, unique mutations of the p53 gene, suggesting a clonal origin. Because these were separate and distinct tumors, we believe this case provides strong support for the differentiation hypothesis. We also provide evidence for markedly different levels of p53 expression in areas with identical p53 mutations.