Low birthweight is associated with the subsequent development of common disorders of adult life, especially hypertension; maternal malnutrition has been suggested as the cause. We suggest an alternative aetiology—increased fetal exposure to maternal glucocorticoids. This hypothesis is supported by our findings that in rats decreased activity of the enzyme that acts as a placental barrier to maternal glucocorticoids (11 β-hydroxysteroid dehydrogenase) is associated with low birthweight. Furthermore, increased exposure of the fetus to exogenous glucocorticoids leads to low birthweight and subsequent hypertension in the offspring. Glucocorticoids acting during critical periods of prenatal development may, like other steroid hormones, exert organisational effects or imprint patterns of response that persist throughout life. Thus, the lifetime risk of common disorders may be partly determined by the intrauterine environment.