Recent studies in Europe, North America, and the developing world have shown that low birth weight and other indices of abnormal fetal growth in babies born at term are linked with a higher prevalence of glucose intolerance and NIDDM in adult life. Reduced fetal growth is also associated with a higher prevalence of the metabolic syndrome (in particular, hypertension and vascular disease) and with insulin resistance in adult life. Because birth size is determined largely by nongenetic factors, these findings have led to the "fetal origins" hypothesis, which proposes that fetal adaptations to an adverse intrauterine environment that reduces fetal growth program lifelong physiological changes. These changes in turn predispose to diabetes and the metabolic syndrome. The mechanisms are unknown, but evidence from animal studies and preliminary human evidence suggests that adverse events in early life may influence the neuroendocrine development of the fetus. This results in long-term alterations in the setpoint of several major hormonal axes, including an increase in adrenal glucocorticoid secretion. These hormonal alterations may contribute to the predisposition to diabetes and the metabolic syndrome in people who were small at birth.