Case summary The case is that of a 42-year-old woman presenting with 45 days of menorrhagia, and no recent history of pregnancy. Ultrasound examination suggested a lower uterine segment polyp, a finding later confirmed at hysteroscopy with curettage. Microscopy revealed diverse fetal tissues, including primitive neuroectoderm, raising the possibility of endometrial teratoma or retained fetal remnants. After extensive sampling and review by specialist gynaecological pathologists the final diagnosis was immature teratoma of the uterine corpus arising within an endometrial polyp. Follow up hysterectomy and surgical cancer staging confirmed metastatic immature teratoma involving multiple intra-abdominal and pelvic sites. Discussion Uterine teratomas are exceptionally rare. As a diverse spectrum of tissues foreign to the uterus can be identified, differentiation from retained fetal tissue is necessary, especially in light of a history of recent pregnancy or abortion. Retained fetal tissue typically encompasses a limited array of tissues, of predominantly neural, cartilaginous and bony origin. Prolonged implantation is not conducive to the survival of most other tissues. Additionally the endometrium shows pregnancy-related changes and inflammation, features not present in our case. Taken together with the variety and viability of the tissues present in our case, retained fetal remnants was less likely. Furthermore, the presence of primitive neuroectoderm and the frankly malignant behaviour exhibited in our case meant that it was best regarded as an immature teratoma. Conclusion Primary endometrial teratomas, although rare, should be considered and differentiated from retained fetal tissue when a diverse constellation of fetal tissues are identified within the endometrium. Thus adequate specimen sampling and clinicopatho-logical correlation is essential.