Endometriosis is a disease of the female reproductive system in which cells similar to those in the endometrium, the layer of tissue that normally covers the inside of the uterus, grow outside the uterus. Lesions can be found on ovaries, fallopian tubes, tissue around the uterus and ovaries (peritoneum), intestines, bladder, and diaphragm; it may also occur in other parts of the body. Some symptoms include pelvic pain, heavy periods, pain with bowel movements, painful urination, and infertility. Nearly half of those affected have chronic pelvic pain, while in 70% pain occurs during menstruation. Pain during sexual intercourse is also common. Infertility occurs in up to half of affected individuals. About 25% of individuals have no symptoms and 85% of those seen with infertility in a tertiary center have no pain. Endometriosis can have both social and psychological effects.

The cause is not entirely clear. Risk factors include having a family history of the condition. The areas of endometriosis bleed each month (menstrual period), resulting in inflammation and scarring. The growths due to endometriosis are not cancer. Diagnosis is usually based on symptoms in combination with medical imaging; however, biopsy is the surest method of diagnosis. Other causes of similar symptoms include pelvic inflammatory disease, irritable bowel syndrome, interstitial cystitis, and fibromyalgia. Endometriosis is commonly misdiagnosed and females often report being incorrectly told their symptoms are trivial or normal. Females with endometriosis see an average of seven physicians before receiving a correct diagnosis, with an average delay of 6.7 years between the onset of symptoms and surgically obtained biopsies, the gold standard for diagnosing the condition. This average delay places endometriosis at the extreme end of diagnostic inefficiency.

Tentative evidence suggests that the use of combined oral contraceptives reduces the risk of endometriosis. Exercise and avoiding large amounts of alcohol may also be preventive. There is no cure for endometriosis, but a number of treatments may improve symptoms. This may include pain medication, hormonal treatments or surgery. The recommended pain medication is usually a non-steroidal anti-inflammatory drug (NSAID), such as naproxen. Taking the active component of the birth control pill continuously or using an intrauterine device with progestogen may also be useful. Gonadotropin-releasing hormone agonist (GnRH agonist) may improve the ability of those who are infertile to get pregnant. Surgical removal of endometriosis may be used to treat those whose symptoms are not manageable with other treatments.

One estimate is that 10.8 million people are affected globally as of 2015. Other sources estimate 6 to 10% of the general female population and 2 to 11% of asymptomatic women are affected. In addition, 11% of women in a general population have undiagnosed endometriosis that can be seen on magnetic resonance imaging (MRI). Endometriosis is most common in those in their thirties and forties; however, it can begin in girls as early as eight years old. It results in few deaths with unadjusted and age-standardized death rates of 0.1 and 0.0 per 100,000. Endometriosis was first determined to be a separate condition in the 1920s. Before that time, endometriosis and adenomyosis were considered together. It is unclear who first described the disease.