Purpose of investigation: Endometrial stromal (ES) tumours may show different types of differentiation including smooth muscle and sex-cord like elements. A common problem is the differential diagnosis of ES tumour variants from other tumours that have smooth muscle or sex cord-like elements. We report a case of collision tumour that was composed of a low-grade endometrial stromal sarcoma (ESS) with sex-cord like differentiation, a uterine tumour resembling ovarian sex-cord tumour (UTROSCT), and a leiomyoma. Result: The patient was a 63-year-old multiparous woman who was referred to us with the complaint of recurrent abnormal uterine bleeding. Magnetic resonance imaging demonstrated a 3.4-cm-dia. solid tumour in her uterine cavity. We performed an abdominal total hysterectomy and a bilateral salpingo-oophorectomy. On gross and microscopic examination, the tumour consisted of a CD10-positive low-grade ESS, a CD10-negative UTROSCT, and a leiomyoma. Conclusion: ESS should be distinguished from UTROSCT and leiomyoma because of its malignant potential.