Type 1 diabetes (T1D) is associated with reproductive dysfunction, particularly in the setting of poor metabolic control. Improvements in contemporary management ameliorates these problems, albeit at the cost of increased exogenous insulin and rising obesity, with emerging reproductive implications.To evaluate changes in body mass index (BMI), and the relationship between obesity, menstrual irregularity and polycystic ovary syndrome (PCOS) in young women with T1D, compared with controls.Longitudinal observational study using data from the Australian Longitudinal Study in Women's Health of the cohort born in 1989-95, from 2013-2015. Three questionnaires administered at baseline and yearly intervals, were used to evaluate self-reported menstrual irregularity, PCOS and BMI.Overall, 15926 women were included at baseline (T1D, n=115; controls, n=15811). 61 women with T1D and 8332 controls remained at Year 2. Median BMI was higher in women with type 1 diabetes (25.5 vs. 22.9 kg/m2 , p<0.001), where over half were overweight or obese (54.4% vs. 32.9%, p<0.001). Median BMI increased by 1.11 and 0.45 kg/m2 , in the T1D and control groups, respectively. T1D was independently associated with an increased risk of menstrual irregularity (RR 1.22, 95%CI 1.02-1.46) and PCOS (RR 2.41, 95%CI 1.70-3.42). Obesity conferred a 4-fold increased risk of PCOS, compared to those with normal BMI (RR 3.93, 95%CI 3.51-4.42).Obesity is prevalent amongst women with T1D, and may be a key contributor to the higher risk of menstrual irregularity and PCOS in this cohort, representing an important opportunity for prevention and intervention.