The polycystic ovary syndrome (PCOS) is a common condition, affecting 10%–15% of women, and is defined by the presence of at least two of the following three criteria (Table 6.1): (1) a menstrual cycle disturbance, that is oligomenorrhoea or amenorrhoea, (2) evidence of hyperandrogenism, as assessed by either physical signs (excess hair growth on the face or body (hirsutism), acne, alopecia) or a biochemical elevation of androgens and/or (3) polycystic ovaries as seen by ultrasound scan, after appropriate endocrine tests have been carried out to rule out other causes of androgen excess and menstrual cycle irregularity. PCOS therefore encompasses many of the natural features experienced by adolescent girls and so it is important to ensure that an appropriate diagnosis is made. Indeed, for this reason, the current guidelines suggest that the diagnosis of PCOS cannot be made until at least 3 years after menarche and some even suggest that one should wait for 8 years, which is when full reproductive maturity has usually been attained.