Babies who are small at birth and during infancy are now known to be at an increased risk of developing coronary heart disease, hypertension and diabetes during adult life. This has led to the suggestion that these diseases are 'programmed' by an inadequate supply of nutrients or oxygen in utero or immediately after birth. The phenomenon of 'programming', whereby undernutrition in early life permanently changes body structure and function, is well documented in animals. As yet we know little about cellular and molecular changes which underlie it, but persisting changes in the secretion of hormones or in the sensitivity of tissues to them may be important in determining adult disease.