PIH, the most common complication of pregnancy, remains a major source of maternal-child morbidity and mortality. Yet the etiology of this disorder is still little understood. There is now a growing body of evidence linking PIH and insulin resistance. Both proteinuric and non-proteinuric PIH predict future essential hypertension, and to a lesser extent, diabetes, disorders strongly related to glucose intolerance and insulin resistance. PIH is associated with diabetes, occurring in up to 50% of diabetic pregnancies. PIH is characterized by the same features that define IRS, including hypertension, dyslipidemia, disruption of endothelial and platelet function and related disturbances of prostanoid synthesis, coagulation and fibrinolytic abnormalities, hyperuricemia, atherosclerotic changes, and obesity. During the last decade, controlled studies by at least 11 different research groups in nine countries have established significant positive associations between both proteinuric and nonproteinuric PIH and various measures of insulin resistance. In particular, prospective investigations by at least five groups of investigators have indicated that relative hyperinsulinemia, glucose intolerance, and insulin insensitivity predict the subsequent development of PIH. These and other studies suggest that insulin resistance may play a causal role in the pathogenesis of PIH, and that some aspects of PIH may represent an early manifestation of IRS, precipitated by the profound metabolic and hemostatic challenges of gestation.