Polycystic ovary syndrome (PCOS) is among the most common endocrine disorders in females, reportedly affecting approximately 5–10% of women of reproductive age. The syndrome is characterized by oligo- or amenorrhoea, with clinical or biochemical hyperandrogenism, as evidenced by acne, hirsutism or alopecia; high serum levels of testosterone, androstenedione and dehydroepiandrosterone (DHEA) and its sulphate (DHEAS) and/or low levels of sex-hormone binding globulin (SHBG) are typical biochemical findings. Lack of internationally agreed diagnostic criteria detract from comparisons between studies reported in the literature, the diagnosis in the USA relying more on abnormal ovarian appearances using ultrasound imaging. By contrast, the diagnosis in the UK rests more heavily on clinical and biochemical features with abnormal imaging not regarded as essential for the diagnosis. Polycystic ovaries on ultrasound are defined by the presence of eight or more subcapsular cysts ≤10 mm and increased ovarian stroma. False-negative scan reports may reflect inter-operator variability while the relatively high prevalence of typical polycystic appearances in asymptomatic women adds to the uncertainties of including ultrasound imaging as a major diagnostic criterion.