Recent studies have shown that reduced fetal growth is associated with the development of the insulin resistance syndrome in adult life. The mechanisms are not known. However increased activity of the hypothalamic-pituitary-adrenal axis (HPAA) may underlie this association; the axis is known to be reset by fetal growth retardation in animals, and there is evidence in humans of an association between raised HPAA activity and the insulin resistance syndrome. We have, therefore, examined the relations among size at birth, plasma cortisol concentrations, and components of the insulin resistance syndrome in a sample of healthy men. We measured 0900 h fasting plasma cortisol and corticosteroid-binding globulin levels in 370 men who were born in Hertfordshire, UK, between 1920-1930 and whose birth weights were recorded. Fasting plasma cortisol concentrations varied from 112-702 nmol/L and were related to systolic blood pressure (P = 0.02), fasting and 2-h plasma glucose concentrations after an oral glucose tolerance test (P = 0.0002 and P = 0.04), plasma triglyceride levels (P = 0.009), and insulin resistance (P = 0.006). Plasma cortisol concentrations fell progressively (P = 0.007) from 408 nmol/L in men whose birth weights were 5.5 lb (2.50 kg) or less to 309 nmol/L among those who weighed 9.5 lb (4.31 kg) or more at birth, a trend independent of age and body mass index. These findings suggest that plasma concentrations of cortisol within the normal range could have an important effect on blood pressure and glucose tolerance. Moreover, this study provides the first evidence that intrauterine programming of the HPAA may be a mechanism underlying the association between low birth weight and the insulin resistance syndrome in adult life.