Endometriosis is a progressive disorder in which tissue, similar to the tissue that normally lines the inside of your uterus, grows outside your uterus. The tissue thickens, breaks down and bleeds with each menstrual cycle. However, because this tissue has no way to exit the body, it becomes trapped. When endometriosis involves the ovaries, cysts form. Surrounding tissue can become irritated, eventually developing scar tissue and adhesions that can cause pelvic tissues and organs to stick to each other. Endometriosis can cause severe pain, especially during menstrual periods, and can also cause fertility problems.