Prolactin serum levels were analysed in 76 infertile patients with endometriosis and 79 infertile patients without endometriosis, and confirmed higher prolactin levels in infertile patients with endometriosis (23.02 ± 1.25 ng/mL vs. 17.21 ± 1.22 ng/mL, respectively) and the correlation of prolactin serum levels with the stage of endometriosis (16.98 ± 1.29 ng/mL for stages I; 18.07 ± 1.50 ng/mL for stages II; 25.59 ± 1.96 ng/mL stages III–IV). This evidence concerns the gene PRL and endometriosis.