Previously overlooked hormone plays critical role in the development of endometriosis Endometriosis, a disease found in up to 10 per cent of women, has been enigmatic since it was first described. A new theory developed by researchers at Simon Fraser University suggests a previously overlooked hormone -- testosterone -- has a critical role in its development. The research could have direct impacts on diagnosis and treatment of the disease, signaling hope for women with endometriosis worldwide. The disease is caused by endometrial tissue growing outside of the uterus, usually in the pelvic area, where it contributes to pain, inflammation, and infertility. But why some women get it, and others do not, has remained unclear.