Polycystic ovary syndrome (PCOS) affects reproductive and cardiometabolic health, yet its pathogenesis remains unclear. Emerging evidence links hemoglobin levels to metabolic disorders, suggesting a potential role in PCOS development. Here, we integrated a large-scale cohort study, Mendelian randomization (A genetic tool to infer causal relationships), bioinformatics analyses, and in vitro experiments to investigate the relationship between hemoglobin levels and PCOS. In a cohort of 20 602 women, each 10 g L