Introduction: Adult obesity is linked with polycystic ovary syndrome (PCOS), but the importance of body size at ages before PCOS is diagnosed is unknown. Objective: To investigate associations between a woman's own birthweight, childhood body mass index (BMI), height and growth patterns in relation to her risk of PCOS. Methods: We included 65,665 girls from the Copenhagen School Health Records Register, born in the period 1960-1996, with information on birthweight and measured weight and height at the ages of 7-13 years. Overweight was defined using International Obesity Task Force (IOTF) criteria. From the Danish National Patient Register, 606 women aged 15-50 years were identified. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated by Cox regression analysis. Results: Birthweight was not associated with PCOS. At the age of 7-13 years, girls with overweight had a higher risk of developing PCOS than girls without overweight; HR 2.83 (95% CI 2.34-3.42) at age 7 years and 2.99 (95% CI 2.38-3.76) at age 13 years. Furthermore, girls with overweight at both 7 and 13 years had a higher risk of developing PCOS than girls without overweight or overweight at only one age. Height was positively associated with PCOS risk at all ages. Girls who were persistently tall or changed from tall to average height had a higher risk of developing PCOS than girls with average height growth. Conclusion: Overweight and tall stature in childhood are positively associated with PCOS risk, but birthweight is not.