Endometriosis is a chronic disease characterized by the growth of endometrial-like tissue beyond the normal confines of the uterine cavity. Most commonly, it is located on the ovaries, but it can also be found on any of the organs inside the pelvic–abdominal cavities. Endometriosis is usually diagnosed at the time of laparoscopic gynecologic surgery although endometriosis cysts (endometriomas) may be presumptively diagnosed on ultrasound.

Although there are several theories about formation of endometriosis, it seems likely that retrograde menstruation (the passage of menstrual debris out of the ends of the fallopian tubes and into the pelvis) plays a major role. Some women may be unable to effectively remove this tissue, allowing lesions to form and grow with continued hormonal stimulation. Since the endometrium sheds through menstrual bleeding every month the endometrial tissue that comprises the endometriosis implants will also respond in kind. This phenomenon leads to inflammation of the pelvic reproductive organs, causing pelvic pain, painful periods (dysmenorrhea), and infertility. Pelvic adhesions or scar tissue may also develop.However, since endometriosis has been described in areas outside of the pelvis (eye, lung, brain, etc.), the retrograde menstruation theory cannot account for all cases of endometriosis.

Endometriosis may be suspected when patients complain of increasingly severe dysmenorrhea, pelvic pain, or infertility, but remember that it can be definitively diagnosed only via surgery. Most often, a diagnostic laparoscopy—a simple outpatient surgical procedure— is used to diagnose endometriosis. Other nonsurgical techniques such as ultrasonography, CT scan, or MRI can occasionally be helpful in their abilities to detect endometriosis.