Endometriosis is a disease in which tissue similar to the lining of the womb grows elsewhere in the body. It occurs in humans and a limited number of other mammals that have a menstruation cycle, notably primates. The tissue most often grows on or around the ovaries and fallopian tubes, on the outside surface of the womb, or the tissues surrounding the womb and the ovaries. It can also grow on other organs in the pelvic region like the bowels, stomach or bladder. Rarely, it can also occur in other parts of the body.

Symptoms can be very different from person to person, varying in range and intensity. About 25% of individuals have no symptoms, while for some it can be a debilitating disease. Common symptoms include pelvic pain, heavy and painful periods, pain with bowel movements, painful urination, pain during sexual intercourse, and infertility. Nearly half of those affected have chronic pelvic pain, while 70% feel pain during menstruation. Up to half of affected individuals are infertile. Besides physical symptoms, endometriosis can affect a person's mental health and social life.

Diagnosis is usually based on symptoms and medical imaging; however, a definitive diagnosis is made through laparoscopy (keyhole surgery). Other causes of similar symptoms include pelvic inflammatory disease, irritable bowel syndrome, interstitial cystitis, and fibromyalgia. Endometriosis is often misdiagnosed and many patients report being incorrectly told their symptoms are trivial or normal. Patients with endometriosis see an average of seven physicians before receiving a correct diagnosis.

Worldwide, around 10% of the female population of reproductive age (190 million women) are affected by endometriosis. Asian women are more likely than White women to be diagnosed with endometriosis. The exact cause of endometriosis is not known. Possible causes include problems with menstrual period flow, genetic factors, hormones, and problems with the immune system. Endometriosis is associated with elevated levels of the female sex hormone estrogen, as well as estrogen receptor sensitivity.

While there is no cure for endometriosis, several 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 hormonal IUD (coil) may also be useful. Gonadotropin-releasing hormone agonist (GnRH agonist) may improve the ability of those who are infertile to conceive. Surgical removal of endometriosis may be used to treat those whose symptoms are not manageable with other treatments.

Pain and infertility are common symptoms, although 20–25% of affected women are asymptomatic. The presence of pain symptoms is associated with the type of endometrial lesions, as 50% of women with typical (peritoneal) lesions, 10% of women with cystic ovarian lesions, and 5% of women with deep endometriosis do not have pain.

A major symptom of endometriosis is recurring pelvic pain. The pain can range from mild to severe cramping or stabbing pain that occurs on both sides of the pelvis, in the lower back and rectal area, and even down the legs. The amount of pain a person feels correlates weakly with the extent or stage (1 through 4) of endometriosis, with some individuals having little or no pain despite having extensive endometriosis or endometriosis with scarring, while others may have severe pain even though they have only a few small areas of endometriosis. The most severe pain is typically associated with menstruation. Pain can also start a week before a menstrual period, during, and even a week after a menstrual period, or it can be constant. The pain can be debilitating and result in emotional stress. Symptoms of endometriosis-related pain may include:

Compared with patients with superficial endometriosis, those with deep disease appear to be more likely to report shooting rectal pain and a sense of their insides being pulled down. Individual pain areas and intensity appear to be unrelated to the surgical diagnosis, and the area of pain is unrelated to the area of endometriosis.