Defining the polycystic ovarian syndrome (PCOS) has challenged clinicians for many years. The clinical, hormonal and morphological definitions of PCOS have their own limitations and do not correspond exactly. Clinically, PCOS can be schematically divided into three components, i.e. hyperandrogenic, anovulatory and dysmetabolic. No one is specific for the syndrome. Hormonally, PCOS has recently been defined by the GnRH agonist test as a functional abnormality in ovarian androgen synthesis. This functional ovarian hyperandrogenism seems closely linked to hyperinsulinism secondary to an insulin resistance. Morphologically, ovarian ultrasonography has emerged in the last decade or so as a new diagnostic tool. However, the sonographic definition of the polycystic ovary (PCO) is controversial, mainly because of a lack of consensus about normative data. The aim of this review is to present the diagnostic dilemma in the diagnosis of PCOS and to discuss the prognostic significance of the PCO.