Objective: The relation between excessive prolactin and endometriosis-related infertility is debatable. Anovulation or defective luteal phase occurs frequently due to hyperprolactinemia in subfertile women. In this investigation, we evaluated the association between serum prolactin levels and the severity of endometriosis. Methods: This retrospective cohort study carried out at the Babol Infertility Research Center looked into the baseline serum prolactin levels of 114 infertile women with endometriosis and compared them to the levels seen in 101 infertile women without endometriosis (controls). Statistical analysis included independent t-test, chi-square, Welch test and ROC curve analysis. Results: Infertile women with endometriosis had significantly higher serum prolactin levels than infertile women without endometriosis ( p =0.003). A significant difference was detected between controls and individuals with endometriosis stages III/IV ( p -value=0.009). Prolactin was found to have diagnostic value to detect endometriosis stages III/IV vs. stages I/II in AUC=0.65, 95% CI (0.55, 0.76). Prolactin values with a cut off set at 20.08 ng/mL had a sensitivity of 0.74 and specificity of 0.54 in detecting disease stages III/IV vs. I/II. The prognostic capability of prolactin in detecting endometriosis in cases vs. controls by ROC curve analysis had an AUC=+0.67, 95% CI (0.60, 0.74). Prolactin values with a cut off set at 17.5 ng/mL had a sensitivity of 0.64 and specificity of 0.63 in segregating subjects with and without endometriosis. Conclusion: Higher prolactin levels were observed in infertile women with more severe endometriosis when compared to infertile women without endometriosis. Prolactin levels act as a probable prognostic biomarker to detect endometriosis stages III/IV vs. I/II and segregate infertile women with endometriosis from subjects without endometriosis.