Pathogenesis of endometriosis involves growth factors, which are synthesized locally. Insulin-like growth factor-1 (IGF-1) prevents apoptosis and has mitogenic action on endometrial cells. The IGF-1 gene undergoes alternative splicing and results in three isoforms (IGF-1Ea, IGF-1Eb, and IGF-1Ec or MGF). We analyzed the mRNA expression of IGF-1 isoforms in tissue samples of eutopic endometrium and endometriotic cyst obtained during laparoscopy from women with endometriosis. We documented that all three IGF-1 isoforms are expressed in both eutopic endometrium and ovarian endometrioma. Furthermore, we documented a significant decrease in all IGF-1 isoform expression in endometriotic cyst compared to endometrium of women with endometriosis. The reduction may correlate with the disease status and presence of fibrotic inactive tissue found in late stages of the disease.