Endometrial decidual reaction in postmenopausal women without an evident exogenous or endogenous progesterone stimulus, is uncommon. The morphological findings are decidual transformation of the endometrial stroma, necrosis, cellular pleomorphism and in some cases hyperplastic glands that exhibit focal atypia raising the possibility of malignancy. We report the case of a 55 years old woman with an idiopathic endometrial decidual reaction. In this woman, immunocytochemistry of endometrial tissue demonstrated the absence of cytoplasmatic and nuclear estrogen receptors. Staining of nuclear progesterone receptors and cytoplasmatic corticoid binding globulin, exceeded the immunoreactivity observed in any period of the menstrual cycle (Rev Méd Chile 2002; 130: 425-429)