Objective: To review all available data and recommend a definition for polycystic ovary syndrome (PCOS) based on published peer-reviewed data, whether already in use or not, to guide clinical diagnosis and future research. Design: Literature review and expert consensus. Setting: Professional society. Patients: None. Intervention(s): None. Main Outcome Measure(s): A systematic review of the published peer-reviewed medical literature, by querying MEDLINE databases, to identify studies evaluating the epidemiology or phenotypic aspects of PCOS. Result(s): The Task Force drafted the initial report, following a consensus process via electronic communication, which was then reviewed and critiqued by the Androgen Excess and PCOS (AE-PCOS) Society AE-PCOS Board of Directors. No section was finalized until all members were satisfied with the contents, and minority opinions noted. Statements were not included that were not supported by peer-reviewed evidence. Conclusion(s): Based on the available data, it is the view of the AE-PCOS Society Task Force that PCOS should be defined by the presence of hyperandrogenism (clinical and/or biochemical), ovarian dysfunction (oligo-anovulation and/or polycystic ovaries), and the exclusion of related disorders. However, a minority considered the possibility that there may be forms of PCOS without overt evidence of hyperandrogenism, but recognized that more data are required before validating this supposition. Finally, the Task Force recognized and fully expects that the definition of this syndrome will evolve over time to incorporate new research findings. © 2009 American Society for Reproductive Medicine.