OBJECTIVES: Endometriosis is defined by the growth of endometrial tissue outside the uterine cavity. It is a common and debilitating benign gynecological condition affecting mainly reproductive-age women with prevalence estimated between 7%-10% of women. The purpose of this study was to understand the cost associated with endometriosis in a secondary care setting in the UK. METHODS: A retrospective analysis was performed using secondary care data from the Cardiff and Vale National Health Service Hospitals Trust including 411 patients diagnosed with endometriosis. A supplementary postal survey was completed by a subset of 80 patients. Data included information on hospital stays, surgical procedures, prescriptions, as well as survey data on primary care treatment, quality of life and productivity loss. Patients were classified into surgical or clinical diagnosis groups depending on type of diagnosis: laparoscopy/laparotomy, or clinician’s assessment only. RESULTS: For hospital visits with endometriosis as primary diagnosis, patients with a surgical diagnosis (n=89) experienced an average hospital stay duration of 0.4 days (SD=0.7) compared to 2.2 days (SD=3.4) for patients with a clinical diagnosis (n=322). The average cost per hospital visit was £871 (surgical group) and £1525 (clinical group). Patients with clinical diagnosis also had more GP visits in the 6 weeks prior to their hospital visit (mean 2.0 (SD=2.9) vs. 1.4 (SD=1.4)). The most common endometriosis-specific pharmacotherapy used in a secondary setting was GnRH agonists (22%) followed by progestins (11%) and oral contraceptives (10%) with drug use in the surgical group exceeding that of the clinical group. Patient utility as measured by the EQ-5D was similar for surgically and clinically diagnosed patients (0.70, SD=0.32 vs. 0.71, SD=0.27). CONCLUSIONS: This study demonstrates that there are substantial costs involved in treating patients in a secondary care setting, with a different pattern of endometriosis-related costs for patients without a confirmed diagnosis of endometriosis.