Endometriosis occurs when endometrial glands and stroma (normal components of the tissue that lines the uterine cavity) implant and grow outside the body of the uterus. Most often, endometriosis is located in the pelvis near or on the reproductive structures. However endometriosis can occur in virtually any tissue of the body, including distant sites like the lung, the knee and the skin.

The two major clinically significant problems associated with endometriosis are infertility and pelvic pain. A common area of misunderstanding is whether these symptoms are linked to one another. Infertility and pelvic pain can and often do occur without the other symptom. That is, a patient can have incapacitating pain due to endometriosis and normal fertility. Also, a patient can have infertility associated with endometriosis with absolutely no pain at all. This fact frequently surprises couples but is commonly seen by infertility specialists.

There are no reliable nonsurgical diagnostic tests for endometriosis. The diagnosis of pelvic endometriosis requires surgery. Ultrasonography shows no specific pattern for pelvic endometriosis, but is often helpful in diagnosing persistent nonfunctioning ovarian cysts that may represent endometriomas (chocolate cysts). Although some specialists seem to think that they can diagnose an endometrioma with certainty using ultrasound, there are abundant scientific reports confirming the nonspecific nature of the ultrasound for this purpose. MRI also has no specific findings, with a reported sensitivity and specificity of only about 60% for the discovery of endometriosis. Blood tests for CA 125 may be elevated with endometriosis, but are also elevated with a tremendous number of other pelvic inflammatory processes so an elevated CA 125 in a reproductive age woman has little value (unless possibly used as a tumor marker when following a patient with known cancer).

Endometriotic lesions (foci of endometriosis) appear to depend upon stimulation by estrogens. The lining of the uterus (endometrium) is one of the most complex tissues in the body in that it continuously changes (grows, modifies its structure, alters its own production of molecular messengers) in response to stimulation by circulating sex steroids. Endometriosis is rarely found in low estrogen environments, such as prior to puberty, after menopause, or following surgical "castration." Endometriosis discovered after menopause is almost always associated with hormone replacement medication containing estrogens.