It is increasingly accepted that alterations of the intrauterine and early postnatal nutritional, metabolic and hormonal environment may predispose individuals to development of diseases in later life. Results from studies of the offspring of diabetic mothers strongly support this hypothesis. It has also been suggested that being light at birth leads to an increased risk of the metabolic syndrome (Syndrome X) in later life (the Barker hypothesis). The pathophysiological mechanisms that underlie this programming are unclear. However, hormones are important environment-dependent organizers of the developing neuroendocrine-immune network, which regulates all the fundamental processes of life. Hormones can act as ‘endogenous functional teratogens’ when present in non-physiological concentrations, induced by alterations in the intrauterine or neonatal environment during critical periods of perinatal life. Perinatal hyperinsulinism is pathognomic in offspring of diabetic mothers. Early hyperinsulinism also occurs as a result of early postnatal overfeeding. In rats, endogenous hyperinsulinism, as well as peripheral or intrahypothalamic insulin treatment during perinatal development, may lead to ‘malprogramming’ of the neuroendocrine systems regulating body weight, food intake and metabolism. This results in an increased disposition to become obese and to develop diabetes throughout life. Similar malprogramming may occur due to perinatal hypercortisolism and hyperleptinism. With regard to ‘small baby syndrome’ and the thrifty phenotype hypothesis, we propose that early postnatal overfeeding of underweight newborns may substantially contribute to their long-term risk of obesity and diabetes. In summary, a complex malprogramming of the central regulation of body weight and metabolism may provide a general aetiopathogenetic concept, explaining perinatally acquired disposition to later disease and, thereby, opening a wide field for primary prevention.