Cornelia de Lange syndrome (CdLS) is a cohesinopathy, which is characterized by multiple structural anomalies as well as mental and growth retardation. A 36-yr-old nulliparous woman with oligomenorrhea was referred to us due to a mass in the uterine corpus. She had been clinically diagnosed with CdLS during infancy based on her specific facial features as well as growth and intellectual retardation. Imaging examinations and an endometrial biopsy revealed endometrial endometrioid carcinoma and polycystic ovary syndrome (PCOS). She underwent a hysterectomy and bilateral salpingo-oophorectomy. The tumor was mainly located at the uterine isthmus and exhibited diffuse exophytic growth. Microscopically, the grade 1 endometrioid carcinoma consisted of extremely well-differentiated glands and showed myometrial invasion. Both swollen ovaries had a thick fibrous cortex and multiple follicles. To the best of our knowledge, this is the first case report of a gynecologic malignancy in an adult patient with CdLS. Several gene mutations have been reported to be causative of CdLS; however, a potential role of these mutations in the pathogenesis of PCOS and subsequent endometrial cancer remains controversial. In this case, PCOS seemed to underlie the endometrial carcinogenesis and then concurrent loss of PTEN and PAX2 expression, confirmed by immunohistochemistry, can facilitate tumor progression. Our case suggests that adult female patients with CdLS can have PCOS and subsequent endometrial carcinoma. As patients with CdLS often have difficulties recognizing and/or reporting menstrual disorder, their care providers should pay particular attention to menstrual cycle irregularities due to the risk of endometrial cancer.