Among theories proposing a non-uterine origin of disease, coelomic metaplasia involves the transformation of normal peritoneal tissue to ectopic endometrial tissue. Agents responsible for such transformation remain poorly defined, although EDCs may be candidates. The closely related induction theory holds that an endogenous inductive stimulus, such as a hormonal or immunologic factor, promotes the differentiation of cells in the peritoneal lining to endometrial cells. Finally, the theory of embryonic Mullerian rests, or mullerianosis, purports cells residual from embryologic Mullerian duct migration maintain the capacity to develop into endometriotic lesions under the influence of estrogen beginning at puberty or perhaps in response to estrogen mimetics. These theories find support in epidemiologic studies reporting a twofold increased risk of endometriosis in women exposed to diethylstilbestrol in utero.

A more recent proposal suggests extra-uterine stem/progenitor cells originating from bone marrow may differentiate into endometriotic tissue. Candidate cell lineages include bone marrow mesenchymal stem progenitors and endothelial progenitors, and this represents an active area of investigation. Support for theories advocating a non-endometrial origin for endometriosis is derived from clinical accounts of histologically confirmed endometriotic tissue in patients without menstrual endometrium, such as individuals with Rokitansky-Kuster-Hauser syndrome and men with prostate cancer undergoing high dose estrogen treatment.

The theory of benign metastasis holds that ectopic endometrial implants are the result of lymphatic or hematogenous dissemination of endometrial cells. Microvascular studies demonstrated flow of lymph from the uterine body into the ovary, rendering possible a role for the lymphatic system in the etiology of ovarian endometriosis. Endometriosis within lymph nodes has been documented in a baboon model of induced endometriosis, and in 6-7% of women at lymphadenectomy. The strongest evidence for the theory of benign metastasis is derived from reports of histologically proven endometriotic lesions occurring in sites distant from the uterus to include bone, lung and brain.