OBJECTIVE: The primary objective was to evaluate the prevalence of undiagnosed iron deficiency in patients with endometriosis. METHODS: We performed a multi-center, cross-sectional study at two tertiary care hospitals. We included 251 non-pregnant women (18-50 years old) presenting with a clinical or surgical diagnosis of symptomatic endometriosis. Patients who consented to the study underwent screening bloodwork (including complete blood count, ferritin, and transferrin saturation) and completed the study survey assessing demographics, medical and surgical history, and validated questionnaires to assess iron deficiency and endometriosis symptoms. RESULTS: The prevalence of iron deficiency in our endometriosis cohort was 53.4% (134/251), and the prevalence of iron deficiency anemia was 13.5% (34/251). Patients with iron deficiency were more likely to have heavy menstrual bleeding (HMB) compared with patients without iron deficiency (66/133, 49.6% vs. 40/115, 34.8%, p = 0.022). Nonetheless, 58% (142/251) of our study population did not endorse HMB. Despite absence of HMB, 47% (67/142) of these patients were iron-deficient. Transferrin saturation was diagnostic for iron deficiency in 63 of 176 patients (35.7%) who had a normal ferritin (≥30 ng/mL). Patients with iron deficiency had a significantly lower adjusted median Functional Assessment of Chronic Illness Therapy Fatigue Subscale score compared with those without iron deficiency (26.3. vs. 29.8, p = 0.025). CONCLUSION: This study highlights the high prevalence of iron deficiency, which remains undiagnosed in over half of patients with endometriosis presenting to a gynecologist. Future research should focus on assessing the effectiveness of iron therapy in improving symptoms and overall well-being in this population.