Lynch syndrome is an autosomal dominant cancer-susceptibility disorder caused by mutations in DNA mismatch repair genes. Women with Lynch syndrome have an increased lifetime risk for endometrial and ovarian cancers. While there is evidence of efficacy for prophylactic surgery, no standard recommendations have been developed to support screening for premalignant endometrial and ovarian epithelial lesions in high-risk women. Here, we report a case of a healthy woman carrying a germline mutation in MLH1 gene with endometrial intra-epithelial neoplasia and ovarian endometriotic lesions exhibiting a loss of MLH1 protein expression. This case report illustrates the malignant potential of endometriosis, and highlights the need for a meticulous management of gynecologic premalignant precursor lesions in reducing cancer risk among related Lynch syndrome women.