Do extrinsic factors including lifestyle, psychosocial factors and healthcare professional engagement independently contribute to weight gain in women with and without polycystic ovary syndrome (PCOS)? Women with PCOS had a higher rate of weight gain than women without PCOS which was most marked in those with unhealthy lifestyles. Women with PCOS have a higher prevalence of overweight/obesity and greater weight gain than women without PCOS. The association of lifestyle factors with weight change in PCOS is not known. The study was a population-based observational study with data collected from seven surveys over 19 years (N = 14 127; Survey 1) involving women with and without PCOS. We used data from the 1973-1978 birth cohort of the Australian Longitudinal Study on Women's Health. Women with PCOS gained more weight annually (0.26 kg/year; 95% CI 0.12, 0.39; P < 0.0001) and over 19 years (4.62 kg; 95% CI 3.04, 6.21; P < 0.0001) than women without PCOS (adjusted analyses). For all women, there were positive associations between weight gain and energy intake, sitting time and stress; inverse associations with fibre intake and physical activity (PA); and no associations with diet quality, glycaemic index, healthcare utilization, depression or anxiety. There were interactions between lifestyle factors (energy intake P = 0.006, glycaemic index P = 0.025, sitting time P = 0.041 and PA P = 0.021), PCOS status and time such that weight gain varied between women with and without PCOS according to these factors. The limitations of this study include the use of self-reported measures such as diet, PA, sitting time, psychological factors and health care utilization. While women with PCOS are more prone to weight gain, lifestyle factors have a more profound impact on weight gain in women with PCOS than without PCOS. These study findings have implications for understanding the mechanisms of weight gain in women with PCOS. They also highlight the importance of early lifestyle intervention as soon as PCOS is diagnosed to address modifiable extrinsic factors and prevent excess weight gain and worsening of the clinical features of PCOS. M.A.A. is funded by the Monash International Tuition Scholarship and Monash Graduate Scholarship and L.J.M. is funded by a National Heart Foundation Future Leader Fellowship. The authors declared no conflict of interest. N/A.