['To determine whether cancer antigen-125 (CA-125) levels are increased in women with endometriosis, especially in those with endometriomas (OMAs), deep infiltrating lesions (DIE), and superficial endometriosis (SUP) compared with controls without endometriosis in a large cohort of operated women.', 'Cross-sectional study (Canadian Task Force classification II-2).', 'Tertiary-care university hospital.', 'Four hundred six women with histologically proven endometriosis and 279 women without endometriosis.', 'Surgical examination of the abdomino-pelvic cavity.', 'Preoperative serum CA-125 antigen levels were evaluated by electrochemoluminescence immunoassay in women with endometriosis and controls. Correlations between serum CA-125 levels and clinical and anatomical characteristics of disease severity were examined. Women with endometriosis displayed higher mean serum CA-125 levels compared with disease-free controls (50.1\xa0±\xa062.4 U/mL vs 22.5\xa0±\xa025.2 U/mL; p\xa0≤\xa0.001). CA-125 levels were significantly increased in women with OMA (60.8\xa0±\xa063.5 U/mL) and DIE (55.2\xa0±\xa068.7 U/mL) compared with women with SUP (23.2\xa0±\xa024.5 U/mL) and controls (22.5\xa0±\xa025.2 U/mL). There was no difference in CA-125 levels between patients with SUP and controls and between patients with OMA and DIE. CA-125 serum levels were correlated with DIE severity: the mean number of DIE lesions and worst DIE lesion.'] Is increased serum cancer antigen-125 a marker for severity of deep endometriosis?