No one knows what causes endometriosis. However, several risk factors have been noted in many women with endometriosis:

Other predisposing factors include previous surgery in which the uterine lining is touched by surgical instruments. It is hereditary. If your mother or sister had endometriosis, there is a good chance that you will develop one too.

The most consistent risk factor is severe stress, especially of a sexual nature. This could be a physical stress or it could be a combination of physical and emotional distress.

No fewer than 12 theories have been proposed since 1921 to explain how and why endometriosis occurs. The reflux menstruation theory was developed by John Sampson, M.D., in 1920. According to this theory, menstrual fluid backs up into the fallopian tubes and drops into the peritoneal cavity, where endometrial cells implant themselves and grow. While this theory offers an answer to the question of what causes endometriosis, it has never been proved. Another popular theory states that endometriosis is caused when endometrial cells spread to other parts of the body through blood and lymphatic channels. Still another theory postulates that endometriosis is in effect a congenital condition.

The "immune defect" theory, is based on the high levels of antibodies (disease-fighting proteins) found in endometriosis patients' blood, plus observation of spontaneous development of endometriosis in monkeys and the fact that women whose close relatives have the disease are at increased risk. In some women a hereditary tendency toward endometriosis, together with an immune reaction against their own tissues, might bring on the disease.

Environmental factors may also contribute to endometriosis. Studies have shown that women who suffer from endometriosis had a higher levels of the pesticide dioxin in their blood. Eighty percent of female rhesus monkeys exposed to very small amounts of these chemicals over four years have developed endometriosis. More of the monkeys on the highest doses had moderate to severe disease than those on the lowest doses. Monkeys not receiving dioxin did not develop either moderate or severe disease.

A 1992 study done in Germany showed that endometriosis is also correlated with PCBs. PCBs and dioxin both interfere with the immune system and the endocrine system. Researchers have postulated that endometriosis is somehow caused by malfunction of both the immune and endocrine systems.