Polycystic ovary syndrome (PCOS) is the most common reproductive disorder in women, yet there is little consensus regarding its aetiology. Here we perform a genome-wide association study of PCOS in up to 5,184 self-reported cases of White European ancestry and 82,759 controls, with follow-up in a further similar to 2,000 clinically validated cases and similar to 100,000 controls. We identify six signals for PCOS at genome-wide statistical significance (P<5 x 10(-8)), in/near genes ERBB4/HER4, YAP1, THADA, FSHB, RAD50 and KRR1. Variants in/near three of the four epidermal growth factor receptor genes (ERBB2/HER2, ERBB3/HER3 and ERBB4/HER4) are associated with PCOS at or near genome-wide significance. Mendelian randomization analyses indicate causal roles in PCOS aetiology for higher BMI (P = 2.5 x 10(-9)), higher insulin resistance (P = 6 x 10(-4)) and lower serum sex hormone binding globulin concentrations (P = 5 x 10(-4)). Furthermore, genetic susceptibility to later menopause is associated with higher PCOS risk (P = 1.6 x 10(-8)) and PCOS-susceptibility alleles are associated with higher serum anti-Mullerian hormone concentrations in girls (P = 8.9 x 10(-5)). This large-scale study implicates an aetiological role of the epidermal growth factor receptors, infers causal mechanisms relevant to clinical management and prevention, and suggests balancing selection mechanisms involved in PCOS risk.