Objective: This study aimed to investigate the risk of Graves’ disease in Polycystic ovary syndrome (PCOS) patients. Design: Population-based retrospective cohort study. Setting: We utilized the medical records randomly selected from the Taiwan National Health Insurance Research Database (NHIRD) during period between 2000 and 2013. Population: We included women aged between 20 and 50 who have no previous Graves’ disease diagnosis records. Methods: We matched the baseline characteristics and the index date of the PCOS-free cohort to the PCOS cohort at a ratio of 4:1 by propensity score matching method. Cox proportional hazards model was used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs). Main outcome measure: To analyze the cumulative incidence and the hazard ratio of Graves’ disease events in both PCOS group and PCOS-free cohort. Results: We identified 5927 patients as the PCOS cohort, and included 23708 patients as the PCOS-free cohort. The PCOS group had a significant increased risk of Graves’ disease with an adjusted hazard ratio (aHR) of 1.34 (95% CI= 1.00-1.81). The p-value was 0.03 in log-rank test. PCOS patients without anxiety, chronic obstructive pulmonary disease, hypertension or hyperlipidemia had a higher risk of Graves’ disease in the stratification analysis. Conclusions: This study revealed a significant association between PCOS and subsequent risk of developing Graves’ disease. Funding: Nil. Tweetable abstract: PCOS increase the risk of Graves’ disease. Keywords: Polycystic ovary syndrome, PCOS, Graves’ disease, GD, hyperthyroidism, autoimmune thyroiditis, cohort, hazard ratio, epidemiology, Taiwan National Health Insurance Research Database