Polycystic ovary syndrome (PCOS) is the commonest cause of anovulatory infertility, menstrual disturbances and hirsutism. In its classic form the presentation is of amenorrhoea or oligomenorrhoea associated with clinical and/or biochemical evidence of hyperandrogenism. However, it is clear that the spectrum of presenting symptoms of women with polycystic ovaries is wide, including anovulation without hirsutism (androgen levels are usually raised) and hirsutism with regular cycles. This has been recognised in the form of a revised diagnostic criteria for PCOS resulting from the joint ESHRE/ASRM consensus conference, held in Rotterdam in 2003. The typical gonadotrophin profile is elevated serum levels of LH with normal or slightly low FSH. PCOS is also associated with a metabolic disturbance in which the central abnormalities appear to be hyperinsulinaemia and insulin resistance. Women with PCOS are relatively hyperinsulinaemic and insulin resistant when compared with weight-matched controls; 2045% of obese PCO subjects have impaired glucose tolerance. Although hyperinsulinaemia and insulin resistance must be regarded as characteristic features of anovulatory women with PCOS, these metabolic abnormalities are not found to the same degree in equally hyperandrogenaemic women with PCO who have regular menstrual cycles. The diagnosis of PCOS is made principally on clinical grounds, supported by ultrasonography and by a small number of biochemical investigations, including (when appropriate) estimation of serum testosterone, LH and FSH. The choice of investigations in women with PCOS depends primarily on the mode of presentation. Because of the high risk of impaired glucose tolerance or frank diabetes, anovulatory women with PCOS should have a glucose tolerance test. It is not necessary to measure circulating insulin levels.