A 42‐year‐old woman underwent partial resection of inoperable stromal endo‐metriosis arising in pelvic endometriosis. Death of massive tumor followed 16 years 5 months later. Analysis of this and 43 reported cases of extra‐uterine Müllerian tumors of all histologic varieties showed 18 within or close to endometriosis. These involved ovary or peritoneum almost exclusively and were diagnosed stromal endometriosis or endometrial sarcoma. Average age was 47 years. Survival time varied widely and averaged 6 years. The remaining 26 cases involved patients with an average age of 57 years, whose tumors arose only in ovary or fallopian tube; 25 were diagnosed carcinosarcoma or malignant‐mixed Müllerian tumor. Death within one year was common. Histologic transitions among endometrial stroma, stromal endometriosis, endometrial sarcoma, and carcinosarcoma support the view that these tumors are closely related and that at least some Müllerian tumors spring from endometrial stromal cells.