Background <p>Letrozole is the preferred treatment for ovulation problems in women with polycystic ovary syndrome (PCOS) because it helps regulate the hormones required for ovulation. Our study examined how genetic differences in the CYP19A1 gene (specifically the rs700518 variation), which controls estrogen production, affect women’s responses to letrozole treatment.</p> Methods <p>We studied 105 Iraqi women (ages 19–33) with PCOS-related infertility at Teba Center from September 2023 to February 2024. Using blood tests and genetic analysis, we tracked hormone levels and physical changes at two key points in the menstrual cycle (days 2 and 12) during letrozole administration. We also examined the DNA to identify different genetic patterns of the CYP19A1 gene.</p> Results <p>Women showed significant changes in hormone levels and physical markers between the beginning and middle of their cycles. Those with the TT genetic pattern had higher estrogen levels (<i>P</i> = 0.03) and a thicker uterine lining (<i>P</i> = 0.02) than those with the other genetic patterns. However, the treatment did not significantly affect other hormones, such as FSH (<i>P</i> = 0.17), LH (<i>P</i> = 0.17), or testosterone, across different genetic groups.</p> Conclusion <p>Our findings show that CYP19A1 genetic makeup significantly affects how a woman responds to letrozole, particularly regarding estrogen levels and reproductive health markers. These findings highlight the potential role of genetic testing in personalizing PCOS fertility treatment, although further validation is needed to confirm its clinical utility.</p>