Introduction: Several cases of low-grade endometrial stromal sarcomas in women with breast cancer have been reported to be associated with tamoxifen therapy. Estrogen receptor expression has been used to characterize the partial estrogenic action of tamoxifen on the endometrium and has been found in tamoxifen-associated endometrial pathologies. Case: A low-grade endometrial stromal sarcoma in a woman with a history of breast cancer treated with adjuvant tamoxifen is presented. Steroid receptor studies performed on the tumor were negative for estrogen and positive for progesterone. Conclusion: The absence of estrogen receptor expression suggests that endometrial stromal sarcomas are not necessarily caused by the estrogenic properties of tamoxifen.