The association between insulin resistance (IR) and the polycystic ovary syndrome (PCOS), as well as that between IR and Type 2 diabetes mellitus (T2DM) is widely recognized. Recently, a growing number of scientific publications are also describing a positive relationship between PCOS and T2DM. The exact nature and importance of this latter relationship is not well understood, although it is widely accepted that IR plays a major role in the development of both disorders. Alternatively, very different and specific tissue/organ defects must be present in order for IR to manifest clinically as either PCOS or T2DM. Understanding whether the nature of this relationship is purely coincidental, as a result of their individual association to IR, versus causal may help to further our understanding of the etiologies of these frequent and morbid conditions. Following we will examine the epidemiology of these associations, and will then review the potential etiologic and pathophysiologic implications of their relationship.