Insulin resistance plays a central role in the pathogenesis of the reproductive as well as metabolic aberrations that characterize the syndrome. Many women with polycystic ovary syndrome (PCOS) manifest a phenotype that overlaps substantially with the so-called "metabolic syndrome" or "syndrome X." These manifestations include obesity, glucose intolerance, hypertension, macrovascular disease, and dyslipidemia. The etiology and manifestations of insulin resistance in PCOS are reviewed in this article.