In this study, we investigated the expression and localisation of the proteins, osteopontin (OPN) and prominin-1 (CD133), as well as the plasma OPN levels in the endometrium of patients with endometriosis. Samples of ectopic endometriotic lesions and normal endometrium were obtained from 31 women with endometriosis and 28 healthy control subjects. The mRNA and protein expression of OPN and CD133 was analysed by real‑time RT-PCR and immunohistochemistry. The plasma levels of OPN were determined by ELISA. Our results revealed that OPN mRNA and protein expression, as well as its release in the blood, was significantly increased in the endometriotic lesions in comparison to normal tissue. Although the presence of CD133+ cells was detected in the normal endometrium, as well as in the endometriosis specimens, a significant quantitative variation of this protein was not demonstrated in the patients with endometriosis. In conclusion, our data indicate that OPN is involved in the development of endometriosis by enhancing the invasiveness, proliferation and survival of endometrial cells in ectopic lesions. CD133 cannot be used as a disease marker for endometriosis, although an involvement of this protein in the pathogenesis of endometriosis cannot be excluded.