Objective: Headache susceptibility (variously described as a “migraine,” “recurrent,” “tension,” “premenstrual,” or “oral contraceptive” headache) is associated epidemiologically with certain gynecologic disorders such as dysmenorrhea, hypermenorrhea, irregular cycles, and ovarian cysts. Autonomic dysfunction such as diarrhea, peripheral vasospasm, and paroxysmal tachycardia are noted in migraineurs (Cady RK, Fox AW. Treating the Headache Patient. Marcel Dekker, Inc. New York, 1995, p. 22). An effort was undertaken to determine if an association exists between headache susceptibility and the occurrence of endometriosis, thereby suggesting that endometriosis also has an autonomic component. Design: Retrospective case analysis. Materials/Methods: A questionnaire (“Autonomic Nervous System and Vascular Complex Questionnaire”) has been developed to evaluate the occurrence of a variety of reported disorders in a female population. Ten questions related to headache susceptibility are included in the general questionnaire. These ten questions were presented to a control group of 24 randomly selected patients who gave no history of spontaneous fetal loss, preterm labor, preeclampsia, or endometriosis. The data from a retrospective review of the same ten questions asked of 23 randomly selected patients with proven endometriosis by laparscopic examination was then recorded. Results: Patients with a diagnosis of endometriosis experienced more frequent headaches (p ≤ 0.025), more severe headaches (p ≤ 0.001), and more premenstrual headaches (p ≤ 0.05). Additionally, this group retired to a dark room more often (p ≤ 0.025) and experienced nausea more often (p ≤ 0.05) with headache occurrence. Chi-squared analysis was used with df = 4. Conclusions: As with other gynecologic disorders, endometriosis appears to occur more frequently in headache susceptible individuals. Endometriosis may be only one of several co-morbid conditions with apparent aberrant autonomic regulation as a common theme, including fibromyalgia, irritable bowel syndrome, and fibrocystic breast disease. This aberrant autonomic regulation could explain a unified concept regarding the origin and perpetuation of endometriosis with its known impact on smooth muscle activity and immune function. Supported By: No support.