Introduction: Uterine tumours most often present as benign pathology of reproductive age women. However, several cases of malignant tumours have been described, which demand a thoroughly pre-operative study in order to decide the correct surgical approach. Case report: A 36 - year - old woman in the postpartum period presents with dysmenorrhea and abnormal uterine bleeding. She was breastfeeding and on progestin-only pill. The ultrasound showed an uterine tumour suggesting a type 4 fibroid (FIGO) with 33.5cm3. Due to the maintenance of symptoms, a new vaginal ultrasound was performed nine months after that revealed a three-fold increase of the tumour size (volume 101.9cm3). Due to the significant increase of the tumour and the persistence of symptoms, four months later the patient underwent an uneventful laparotomy with removal of the uterine tumour. The histopathological study of the operative specimen revealed a smooth muscle tumor of uncertain malignant potential. The patient was reoperated to complete the treatment and adequated staging. The result of the histopathological study of the uterus revealed an endometrial stromal sarcoma (ESS). The histopathological study revealed follicular cysts in both ovaries and the cytological study of ascitic fluid was negative for malignant cells. The patient is in the fourth year of follow-up and undergoing tamoxifen therapy and presents a normal CA 125 and a breast ultrasound BIRADS 1. Discussion: ESS are less than 10% of uterine sarcomas and approximately 1% of malignant uterine tumors. Regarding their clinical presentation, a benign leiomyoma is frequently suspected. It is not possible to reliably distinguish between a leiomyoma and uterine sarcoma based on symptoms. In cases where a new uterine tumor arises, it should always be re-evaluated, within a period of no more than 6-9 months for evaluation of growth and modification of characteristics. In this case, the onset of symptoms in the postpartum period when the patient on progestin-only pill were perhaps suspicious but nevertheless the first ultrasound was reassuring of benignity.ESS metastasizes in 40-50% of cases to the abdomen and pelvis, 25% to the lung and more rarely to the spine and spinal cord. With regard to the prognosis of ESS, it has a global survival of 65-76% at 10 years. Compared with the other types of endometrial sarcomas, ESS has a good prognosis, together with adenossarcoma which was also the case of our patient.