Spontaneous and steroid-induced recurrence of endometriosis after suppression by a gonadotropin-releasing hormone antagonist in the rat. Recurrent endometriosis in women is difficult to study because of the ethical consideration of performing repeated surgeries. Previously in the rat model we described therapeutic regression of endometriosis with the gonadotropin-releasing hormone antagonist antide. Presently we report the spontaneous and steroid-induced recurrence of endometriosis after withdrawal from antide therapy. Rats with endometriosis received antide or vehicle on days 0 (proestrus), 3, 6, and 9 and were killed on days 0, 6, 12, 18, 24, 30, and 42 (n = 4 antide-treated and 4 vehicle-treated rats killed per day). Additional antide-treated rats (n = 4 per treatment) received estrogen, progesterone, both estrogen and progesterone, cholesterol, and no steroid on day 9 and were killed on day 12. Antide significantly suppressed endometriotic implant size on days 12, 18, and 24. However, implant size spontaneously returned to pretreatment values by day 30. Administration of steroids on day 9 elicited regrowth of antide-suppressed endometriosis (estrogen plus progesterone greater than estrogen, progesterone, or cholesterol greater than no steroid) by day 12. This resilience of endometriosis offers an explanation for treatment failure and recurrence of the disease in women. 