Endometriosis A condition resulting from the appearance of endometrial tissue outside the womb and causing pelvic pain, especially associated with menstruation. Endometriosis is usually a long-lasting (chronic) disease. When you have endometriosis, the type of tissue that lines your uterus is also growing outside your uterus. The clumps of tissue (called implants) may have grown on your ovaries or fallopian tubes, the outer wall of the uterus, the intestines, or other organs in the belly. In rare cases they spread to areas beyond the belly.