Objective: The Androgen Excess Society (AES) charged a task force to review all available data and recommend an evidence-based definition for polycystic ovary syndrome (PCOS), whether already in use or not, to guide clinical diagnosis and future research. Participants: Participants included expert investigators in the field. Evidence: Based on a systematic review of the published peer-reviewed medical literature, by querying MEDLINE databases, we tried to identify studies evaluating the epidemiology or phenotypic aspects of PCOS. Consensus Process: The task force drafted the initial report, following a consensus process via electronic communication, which was then reviewed and critiqued by the AES Board of Directors. No section was finalized until all members were satisfied with the contents and minority opinions noted. Statements that were not supported by peer-reviewed evidence were not included. Conclusions: Based on the available data, it is the view of the AES Task Force on the Phenotype of PCOS that there should be acceptance of the original 1990 National Institutes of Health criteria with some modifications, taking into consideration the concerns expressed in the proceedings of the 2003 Rotterdam conference. A principal conclusion was that PCOS should be first considered a disorder of androgen excess or hyperandrogenism, although 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. Copyright © 2006 by The Endocrine Society.