In contrast to previous reports describing higher aromatase levels in the eutopic endometria of patients with endometriosis than in the endometrium from disease-free women, as detected by using RT-PCR and immunochemistry [21, 41], we report significantly lower levels of the CYP19A1/aromatase mRNA and protein in the eutopic endometrium of women with diagnosed OE in a menstrual phase-specific manner, regardless of their fertility status. The gene discussed is CYP19A1; the disease is endometriosis.