What is the incidence/prevalence of type 2 diabetes in women with polycystic ovary syndrome (PCOS) and the economic burden associated with PCOS in the UK?The incidence and prevalence of type 2 diabetes in women with PCOS are 3-33 per 1000 person years and 26.5%, respectively, with an associated annual healthcare burden of at least £237 million in the UK.Although observational studies have been designed to assess the incidence of diabetes in women with PCOS, these have been open to criticism because of short periods of follow-up, small sample sizes or invalidated diagnosis of PCOS. Only one study has estimated the healthcare-related economic burden of PCOS, reporting a cost of $4.36 billion per year in the USA.This was a modelling study using individual patient data from a UK primary care database between 2004 and 2014 and aggregate data from the literature to obtain conversion rates through disease progression of PCOS. A simulation approach was applied to model the population dynamics of PCOS over a follow-up period of 25 years.A total of 14 135 women with PCOS or symptoms indicative of PCOS were selected from the primary care database to estimate the incidence of confirmed diagnosis of PCOS and diagnosis of type 2 diabetes. A ‘virtual’ cohort including the entire PCOS population (size estimated from the UK census data) was simulated to model the population dynamics of PCOS. The economic and utility analyses were further conducted from a healthcare perspective.The peak conversion rate from possible to diagnosed PCOS was 121 per 1000 person-year (PY). The maximal incidence of type 2 diabetes was 33 per 1000 PY. The estimated prevalence of diabetes in the PCOS population was 26.5% (95% interval: 25.4-27.8%) during a 25-year follow-up. The annual healthcare burden of PCOS based on our conservative estimate is at least £237 million for the follow-up period examined.Due to lack of data, a full economic evaluation including healthcare costs of all the comorbidities associated with PCOS was not possible. Simplification of the real-world situation represented by the model may be a concern.This study suggests that a large number of women with symptoms indicative of PCOS never receive a definitive diagnosis yet can suffer from a rapid conversion to diabetes. This significantly reduces the quality of life for individual patients and incurs high costs for healthcare providers. As the risk of diabetes in women with PCOS is similar to that seen in populations at high risks of diabetes, it is possible that including them in national screening programmes may be cost effective.There was no funding for the current study.