Endometriosis is a multifactorial disease that is manifested by infertility and pelvic pain. The purpose of the study was to evaluate the effect of progesterone treatment on the serum level of osteopontin, a multipotent cytokine, in patients with endometriosis. The study was prospective and we evaluated osteopontin levels that were measured in the serum of 40 patients with endometriosis and 12 healthy women using a standardized Enzyme-Linked Immunosorbent Assay (ELISA) kit. Osteopontin seric levels were lower in endometriosis patients and increased after progesterone treatment. Because of the large dispersion of data even in the control group, we find the association between osteopontin and endometriosis questionable.