The rarely occurring disease of endolymphatic stromal myosis is discussed by means of an impressive case. The cells of this semimalignant tumour resemble those of the endometrium, but it is not clear whether this tumour can be derived histogenetically from the endometrium or from the cells of the duct of Müller (ductus paramesonephricus) or from pericytal cells. The histological pattern of the tumour, which is characterized by the uniformity of small spindle-like oval cells without remarkable mitotic activity, and by a lack of atypical criteria, does not permit any prognosis in respect of its clinical behaviour. According to the rare references in literature, the clinical course is usually protracted with occasional occurrence of metastases and relapses. The therapy consists of radiotherapy and administration of progesterone. The case described in this article is a clinically malignant one because of disseminated metastases and relapse following initially successful therapy.