Lymphangioleiomyomatosis (LAM) is a destructive lung disease of women caused by proliferation of neoplastic-like LAM cells, with mutations in the TSC1/2 tumor suppressor genes. Based on case reports, levels of cancer antigen 125 (CA-125), an ovarian cancer biomarker, can be elevated in patients with LAM. We hypothesized that elevated serum CA-125 levels seen in some patients with LAM were due to LAM, not other malignancies, and might respond to sirolimus treatment. Serum CA-125 levels were measured for 241 patients at each visit. Medical records were reviewed for co-morbidities, disease progression, and response to sirolimus treatment. CA-125 expression in LAM cells was determined by using immunohistochemical analysis. Almost 25% of patients with LAM had at least one elevated serum CA-125 measurement. Higher serum CA-125 levels correlated with lower FEV1, premenopausal status, and pleural effusion in a multivariate model (each P < .001). Serum CA-125 levels decreased following sirolimus treatment (P = .002). CA-125 and α-smooth muscle actin were co-expressed in LAM lung nodules. Higher serum CA-125 levels were associated with pleural effusions and reduced pulmonary function and were decreased with sirolimus therapy. LAM cells express CA-125. Some elevated serum CA-125 levels may reflect serosal membrane involvement. Copyright © 2017. Published by Elsevier Inc.