Polycystic ovary syndrome (PCOS) affects 6-18 of women. The natural history of weight gain in women with PCOS has not been well described. Here we aimed to examine longitudinal weight gain in women with and without PCOS and to assess the association between obesity and PCOS prevalence. Design and Methods The observational study was set in the general community. Participants were women randomly selected from the national health insurance scheme (Medicare) database. Mailed survey data were collected by the Australian Longitudinal Study on Women s Health. Data from respondents to survey 4, aged 28-33 years (2006, n = 9,145) were analyzed. The main outcome measures were PCOS prevalence and body mass index (BMI). Results Self-reported PCOS prevalence was 5.8 (95 CI: 5.3 -6.4 ). Women reporting PCOS had higher weight, mean BMI [2.5 kg/m2 (95 CI: 1.9-3.1)], and greater 10-year weight gain [2.6 kg (95 CI: 1.2-4.0)]. BMI was the strongest correlate of PCOS status with every BMI increment increasing the risk of reporting PCOS by 9.2 (95 CI: 6 -12 ). Conclusions This community based observational study with longitudinal reporting of weight shows that weight, BMI, and 10-year weight gain were higher in PCOS. We report the novel finding that obesity and greater weight gain are significantly associated with PCOS status. Considering the prevalence, major health and economic burden of PCOS, the increasing weight gain in young women, and established benefits of weight loss, these results have major public health implications.